This work was supported by the Consejo Nacional de Ciencia y Tecnolog&#237;a (CONACyT), CB-256767. The authors thank Isaac Mor&#225;n for his technical assistance and Ana Escalante from the Computer Unit of the Instituto de Fisiolog&#237;a Celular (IFC) for her valuable assistance.

The experiments were approved by the Bioethical Committee of the Institute of Cellular Physiology of UNAM (No. CECB_08) and carried out under the guidelines of The Code of Ethics of the World Medical Association.
1. Experimental Set-up
<ol>
	<li>Material and stimuli set-up for performing an aperiodic interval discrimination (AID) task
	<ol>
		<li>Perform this experiment on a computer with a minimum of 8 GB RAM, 2.5 GHz processor, and a 60 Hz refreshing rate monitor to create and run the task.</li>
		<li>Obtain a set of noise-canceling headphones to avoid environmental sounds distracting the participants while performing the task.</li>
		<li>Use a decibel meter to set the volume of the headphones to ~65 dB SPL.</li>
		<li>Create the V, A, and AV stimuli for the task by running a graphical user interface (GUI) included in this protocol (Figure 1), or by using programs such as PsychToolbox or PsychoPy.
		<ol>
			<li>Download the Stimuli_GUI.zip file from http://www.ifc.unam.mx/investigadores/Luis-lemus. Then, open MATLAB (2016a or higher for this GUI).
			<ol>
				<li>Click on the SetPath option at the MATLAB&#8217;s menu tab to add the Stimuli_GUI folder to the workspace. First, select the Add Folder button, select the Stimuli_GUI folder, and press the Save button. Finally, close the window by clicking on the Close button.</li>
				<li>Open the Stimuli_GUI.m file using the Open option under the Main menu tab. Then, press F5 on the keyboard to display the GUI (Figure 1).</li>
			</ol>
			</li>
			<li>Click on the Condition popup menu to select the preferred distribution of pulses (i.e., Periodic for creating a stimulus of equidistant pulses, or Aperiodic for a random distribution). Then, select the desired number of pulses (i.e., 2 - 6) in the Number of Pulses popup menu. Finally, enter the desired duration of the stimulus in the Duration dialog box. 
			CAUTION: To avoid V flicker fusion (i.e., two or more pulses are perceived as only one), it is essential to create interpulse intervals (IPI) of a minimum of 30 ms. Therefore, given that each of the pulses is predefined to last 50 ms, the maximum number of pulses in a stimulus is constrained by the IPIs. Pulses exceeding the minimum IPI produce an error. 
			Note: The program generates images in an Audio Video Interleave (AVI) format at a rate of 60 frames per second. However, they can be created online at each trial using PsychToolbox or PsychoPy. Consider creating 50-ms pulses by concatenating at least three frames of 4&#176; gray circles on black backgrounds. Here, the method generates AVI and WAV files for implementing in LabVIEW, thus suggesting the possibility of using more complex video or audio clips.</li>
			<li>Click on the Generate IPI button to display the IPIs&#8217; values at the IPI values box and to see a plot of the resultant distribution of the pulses. 
			Note: The IPI values update automatically at every click of the Generate IPI button. These values can be copied and saved for further analysis.</li>
			<li>Generate and store a V stimulus by typing a descriptive filename (e.g., PeriodicVisual500ms.avi) in the Enter Video Filename dialog box. Click on the Generate Video button and wait for the popup window displaying gray ~4&#176; circles to close. Then, click on the Play button to see the created V stimulus. 
			CAUTION: While the program generates the images, do not click on other figures, as this may cause the program to lose the figure&#8217;s handle and produce a faulty video. 
			Note: The angular amplitude of a V object is obtained by the following equation (2). 
			<img alt="Equation 2" src="/files/ftp_upload/58408/58408eq2.jpg" style="opacity: 0.9;" />(2) 
			&#8203;Here, a is the amplitude of V expressed in degrees, S is the visual&#8217;s size in centimeters, measured at the screen, and D is the distance in centimeters from the observer to the screen.</li>
			<li>Generate and store an A stimulus using the same V IPI values by typing a descriptive filename (e.g., AperiodicAcoustic500ms.wav) in the Enter Audio Filename dialog box. Click on the Generate Audio button to observe a plot of the created audio and click on the Play button to listen to the new audio. 
			Note: The predefined A pulse frequency is 1 kHz; however, it is possible to change it in the Sound Frequency (Hz) dialog box.</li>
			<li>Repeat steps 1.1.4.2 through 1.1.4.5 to create 10 aperiodic (AP) stimuli for each of the comparison intervals of the AID task (i.e., V and A intervals from 500 ms to 1,100 ms in steps of 100 ms). Create only one periodic (P) stimulus for each of the intervals of the control sets.</li>
		</ol>
		</li>
		<li>Generate an extended white-noise clip (e.g., 30 min) to use as background during the experiment, or download it from an internet library.</li>
		<li>Create a 3&#176; white cross and save it in a JPEG file to use as a cue for the participants to initiate a trial. 
		Note: AV stimuli result from superimposing V and A congruent clips during the execution of the task. Shift the A stimuli up to 90 ms after the V onset for producing perceptual simultaneity32.</li>
	</ol>
	</li>
	<li>Task design and implementation
	<ol>
		<li>Create sets of P and AP V, A, and AV trials by listing the names of the created stimuli in an Excel sheet. Use different columns to include all the information required during the task, such as the modality of the reference and comparison stimuli, the number of repetitions per trial, the stimuli durations, and the expected response (see an exemplary CSV file included in Stimuli_GUI.zip file). Save each of the sets in a comma-separated values (CSV) format. 
		Note: The experiment aims to obtain psychometric functions of the probability of perceiving the test stimuli (i.e., the comparison stimuli) longer than the reference as a function of the variations of the comparison stimuli. Therefore, trials to generate psychometric functions must employ a reference stimulus fixed at half of the range of intervals (i.e., 800 ms). However, to guarantee that the participants&#8217; criteria rely on the reference stimulus, they must always attend to both the reference and the comparison. Therefore, trials of varying references should be included to counterbalance the number of varying comparisons. Finally, consider presenting blocks of V, A, and AV trials to avoid attentional effects. However, always present P and AP trials randomly intercalated.</li>
		<li>Create a program for automatically running the task using PsychToolbox or PsychoPy, or download and run the automatized 2IFC_Task available at http://www.ifc.unam.mx/investigadores/Luis-lemus (for running in LabVIEW 2014 or higher).
		<ol>
			<li>Open the 2IFC_Task by double-clicking on the task&#8217;s file.</li>
			<li>Load the created stimuli by selecting the stimulus folder from the Control Panel. First, use the up and down buttons on the dialog box to display a 0. Then, press the folder icon to select the stimulus folder.</li>
			<li>Repeat step 1.2.2.2 to set the File Path to 1, 2, 3, or 4 to load the CSV set of trials file, a TXT output file, a WAV background audio, and a cue of a white cross in JPEG format, respectively. 
			Note: When programming a task, store data in a convenient format for offline analysis (e.g., in TXT or CSV format). Include information about the trial: the order of appearance and the behavioral outcomes, such as hits, errors, reaction times, and response times.</li>
			<li>Press the White Noise button located on the Control Panel to activate the background noise. Then, place the decibel meter as near as possible to the headphones and set the OS volume control to ~65 dB SPL. Finally, adjust the Background Volume control located on the Control Panel to ~55 dB SPL.</li>
			<li>Use the dialog boxes Pre_S1 and Inter_Stim to specify time-lapses of the first stimulus delivery and of the interstimulus separation, respectively. 
			Note: The default times are 1,000 ms. Other graphical indicators are for the examiner to observe results in real-time (e.g., a bar plot of the performance per condition and displays of the number of hits, errors, false alarms, and a current number of trials).</li>
			<li>Test the task by clicking on the Run right arrow icon under the Tools tab and perform some test trials. 
			Note: We recommend to use two monitors, one for delivering the task and the other for monitoring the task online.
			<ol>
				<li>Initiate each trial by holding the spacebar down after the appearance of the visual cue at the center of the screen. Release the spacebar after the delivery of a pair of stimuli and press the upward or the downward arrow key to finalize the trial.</li>
				<li>Repeat step 1.2.2.6.1 until the set is completed. The task stops automatically. Alternatively, abort the task by clicking on the Stop button in the Control Panel menu.</li>
			</ol>
			</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
2. Participants
<ol>
	<li>Recruit 10 to 30 male and female right-handed participants, with no more than ten years of difference in age between them, with normal or corrected-to-normal vision, and no auditory deficits.</li>
	<li>Ask the participants to fill out a questionnaire regarding their age, gender, handedness, and physical or psychological conditions (e.g., having visual or auditory deficits, musical training, and drug intake).</li>
	<li>Tell the participants about the aim, procedures, and duration of the experiment. Be careful no bias is induced (e.g., telling them about the occurrence of P or AP conditions). Then, ask the participants to give written consent to participate in the experiments.</li>
</ol>
3. Experimental Procedure
<ol>
	<li>Perform the experiments in a quiet room with constant lighting.</li>
	<li>Run the task.</li>
	<li>Calibrate the decibel meter and repeat the procedure as described in step 1.2.2.4. 
	CAUTION: Throughout the experiment, acoustic stimuli should be delivered binaurally at ~65 dB SPL. It is essential to use a decibel meter to test the acoustic amplitudes before the experiment to avoid injuries.</li>
	<li>Ask the participant to sit comfortably in front of the monitor, situated at a distance of 60 cm. Then, place the keyboard at a reachable distance, and adjust the headphones to the participant&#8217;s head (Figure 2C).</li>
	<li>Instruct the participant to start a trial after the appearance of the visual cue by pressing and holding down the spacebar for the entire trial. Indicate to the participant to release the spacebar after the presentation of two sequential stimuli and to press the upward arrow key if the second stimulus lasted longer than the first, or to press the downward arrow key if it lasted for a shorter period of time (Figure 2B).</li>
	<li>Finally, instruct the participant to use only the right index finger to complete the task, and comment on the possibility of taking a 5-min break in case the participant feels tired or distracted during the experiment.</li>
	<li>Turn on the noise-blocking feature of the headphones and let the participant practice 10 - 15 trials. 
	Note: During this phase, providing a visual input for correct answers is recommended. Also, it is possible to provide feedback during the experiment; however, be aware of possible biases.</li>
	<li>Run the task.</li>
</ol>
4. Data Analysis
<ol>
	<li>Calculate the mean and the standard error of the mean of the performance of each of the blocks of P and AP, V, A, and AV trials.</li>
	<li>Generate scatter plots of the probability of perceiving the comparison stimulus longer than the reference as a function of the comparison&#8217;s intervals. Then, fit a logistic function to the data. 
	Note: As noted in the Introduction, selecting a convenient model depends on the experiment and the data. An example of a model is the tanh as reported for the AID task. Such a model delivers four parameters (inset in Figure 3A) defined by: 
	<img alt="Equation 3" src="/files/ftp_upload/58408/58408eq3.jpg" style="opacity: 0.9;" /> 
	The parameter a corresponds to the performance&#8217;s magnitude measured from the inflection point to the plateau. The &#946; parameter corresponds to the first derivative at the inflection point. The higher the value, the easier to perceive a transition between longer and shorter, when compared to the reference categories. The parameter &#952; or X0 is the abscissa value of the projection of the inflection point (i.e., the point of subjective equality). The shifting of such a parameter represents overall temporal biases. Finally, c or Y0 represents the inflection point at the ordinate and reveals biases toward a particular response. Alternative routines for fitting and analyzing psychometric functions are the Palamedes toolbox6 and quickpsy33.</li>
	<li>Repeat the procedure from step 4.1 for analyzing the reaction times and response times.</li>
	<li>Perform statistical analyses to compare the P and AP accuracy distributions within each of the sensory modalities.</li>
	<li>Perform additional analyses, such as Pearson correlations, to find the relationship between periodicity indices and accuracy and between periodicity indices and reaction times.</li>
</ol>

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H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Neural+Basis+of+the+Perception+and+Estimation+of+Time.">Neural Basis of the Perception and Estimation of Time.</a> Annual Review of Neuroscience. 36 (1), 313-336 (2013).</li><li>Chandrasekaran, C., Lemus, L., Ghazanfar, A. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dynamic+faces+speed+up+the+onset+of+auditory+cortical+spiking+responses+during+vocal+detection.">Dynamic faces speed up the onset of auditory cortical spiking responses during vocal detection.</a> Proceedings of the National Academy of Sciences of the United States of America. 110 (48), E4668-E4677 (2013).</li></ol>

The authors have nothing to disclose.

In psychophysics, the selection of a task depends on particular interests in perceptual phenomena5,6. For instance, this protocol consisted of recreating a previously reported paradigm on the time-interval perception of visual, auditory, and audiovisual stimuli of aperiodically arrayed pulses, which implemented the 2IFC method13. Here, as in most of the psychophysics tasks, adequate hardware and software are essential to create, reproduce,...

The purpose of this protocol is to convey a procedure for a standard experiment on psychophysics. Psychophysics is the study of perception phenomena through the measure of behavioral responses, elicited by sensory inputs1,2,3. Usually, human psychophysics is an inexpensive and essential tool to implement in imaging or neurophysiological experiments4. However, it is never easy to select the most appropriate psychophysical method out of many that exist, and the selection somewhat depends on experience and preference. Nevertheless, we encourage beginners to revise available methodologies thoroughly in order to learn about selection criteria5,6,7. Here, we provide a procedure for performing a 2IFC task, which many researchers frequently use for studying perceptual processes such as working memory8, decision making9,10, or time perception11,12,13.
To guide the readers along the method, we recreate a report on the perceptual duration of visual (V), auditory (A), and audiovisual (AV) intervals of aperiodic sequences of pulses. We will refer to this task as an aperiodic interval discrimination (AID) task13. When attempting to describe this paradigm in psychophysics jargon, it would be a class-A, type-1, performance-based, criterion-dependent discrimination task that uses a non-adaptive method of the constants and a hyperbolic tangent (tanh) model to calculate a differential threshold. Even when such a characterization sounds somewhat entangled, we will use it to introduce the reader to some general aspects of psychophysics, hoping to provide decision criteria for new experiments and maybe even the possibility of tailoring the current protocol to other needs.
Any psychophysical experiment, such as a 2IFC task, requires implementing stimuli, a task, a method, an analysis, and a measurement6. The goal is to obtain the psychometric function that better accounts for the measured performance14. A 2IFC task consists of presenting to participants, who are na&#239;ve to the purpose of the experiment, trials of two sequential stimuli. After comparing the stimuli, they report the outcome by selecting one, and only one, out of two possible responses that better suits their perception.
With stimuli, we refer to technical considerations about the sensory modality under study. A class-A experiment consists of the comparison of stimuli of the same modality within a trial, whereas class-B experiments include cross-modal comparisons. Other essential considerations about stimuli include their implementation, such as the technical ways of modulating stimuli within a required range. For example, if we want to find the just noticeable difference (JND) between two flutter frequencies vibrating on the skin15, we need a precision stimulator to generate frequencies within the confines of flutter (i.e., 4 - 40 Hz). In other words, the dynamic operating range of the technical elements depend on the dynamic spectrum of each sensory modality.
Selecting a task is about the perceptual phenomenon under study. For instance, finding whether two stimuli are the same, or equivalent, may rely on different brain mechanisms than those resolving if a stimulus is longer or shorter than a reference16 (as in the AID paradigm). Intrinsically, stimuli selection defines the type of obtained responses. Type-1 experiments, sometimes closely related to the so-called performance experiments, include correct or incorrect responses. In contrast, a type-2 experiment (or appearance experiment) produces mostly qualitative answers that depend on the participant&#39;s criteria and not on any explicitly imposed criterion; in other words, criterion-independent experiments. It is noteworthy that 2IFC task responses are criterion dependent because, in every trial, the standard stimulus (sometimes called base or reference stimulus) constitutes the criterion on which the comparison&#39;s perception depends.
The method may refer to three things; first, it may refer to the mechanism for selecting the range of stimuli to test or, in other words, to an already known range of stimulus variability, as opposed to adaptive methods aimed to establish the adequate range17. These adaptive matters are recommended for quickly finding detection and discrimination thresholds and for minimum trial repetitions18. Also, adaptive methods are optimal for pilot experiments. The second definition of a method is the scale of stimuli modulations (e.g., the method of the constants) or a logarithmic scale. The selected scale may or may not be a direct consequence of the outcome of an adaptive method, but primarily, it regards the dynamics of the studied sensory modality. Lastly, the method also refers to the number of trials and their presentation order.
As for analysis, it relates to the statistics of experimental measurements. Regardless of selecting appropriate analytical methods for comparisons between test and control groups, psychophysics is mostly about measuring absolute or differential thresholds between two conditions (e.g., presence vs. absence of a stimulus, or the JND between two stimuli), particularly in 2IFC19. Such measurements derive from psychometric functions (i.e., continuous models of behavior as a function of the probability of detecting or discerning one of the conditions at stake). Selecting the model function depends on the scale or, in other words, on the spacing of the values of the independent variable. Functions such as cumulative normal, logistic, Quick, and Weibull are appropriate for values spaced linearly, whereas Gumbel and log-Quick are better suited for logarithmic spacing. Alternative models also exist, such as the tanh employed in the AID task. Importantly, selecting a correct model depends on the parameters of interest, as considered in the design of the experiment20. After fitting the data to a model, it should be possible to derive two parameters: &#945; and &#946; parameters. In the case of a logistic function typically employed in a 2IFC paradigm, &#945; refers to the abscissas value projecting to the point of subjective equality (i.e., at half the logistic). The &#946; parameter refers to the slope at &#945; value (i.e., the steepness of the transition between conditions). Finally, a parameter commonly obtained out of a psychometric curve is the differential limen21 (DL). In a 2IFC experiment, the DL relates to &#946;, but strictly, corresponds to the minimum perceived difference between two intervals. The formula to determine the DL is the following equation (1).
<img alt="Equation 1" src="/files/ftp_upload/58408/58408eq1.jpg" />(1)
Here, x stands for independent variable values projecting at a 0.75 and 0.25 performance measured directly at the sigmoidal curve. Up until this point, we have covered only some generalities about psychometric functions. We recommend further study of estimating and interpreting psychometric functions, with these and other parameters22.
Other technical aspects to consider when implementing a psychophysical experiment are related to equipment and software. Memory and speed capacities of commercial computers nowadays are usually optimal for processing in high-fidelity visual and auditory tasks. Moreover, the dynamic resolution of complementary material, such as noise-blocking headphones, speakers, and monitors, must fulfill the sampling rate at which the sensory modalities operate (e.g., frequency, amplitude, contrast, and refreshing rate). Also, software programs such as PsychToolbox23 and PsychoPy24 are easy to implement and highly efficient at synchronizing tasks&#39; events and equipment.
The previously described AID task assembles many of the topics described above for a 2IFC paradigm. Interestingly, it explores the perception of V, A, and AV intervals in the range of milliseconds, where most of the brain&#39;s processes occur25,26,27. Paradoxically, it is also a challenging lapse for studying vision, which, compared to audition, begets a somewhat constrained sampling rate28. In this sense, multimodal comparisons require additional theoretical scopes12,29,30. Sometimes, they need further tailoring to encompass a common modulation spectrum or to achieve congruent interpretations.
This protocol focuses on a discrimination task (i.e., a 2IFC where a base stimulus, also called reference or standard, is contrasted against a set of comparison or test stimuli to find a JND or, in other words, a discrimination threshold). Here, the task is set to study the capacity of humans to discriminate time intervals of V, A, or AV aperiodic patterns of pulses13. We provide information on creating and parameterizing stimuli, as well as on analyses of accuracy and reaction times. Importantly, we discuss how to interpret subjects&#39; time perception from the psychometrics statistical outcome parameters, and some experimental and analytical alternatives within topics of a 2IFC psychophysical method.

<table><tbody><tr><td>Lapt top Dell Precision</td><td>Dell</td><td>M6800 CTO</td><td>Procesador Intel Core i7-4710MQ, 2.5GHz RAM 16 GB, 64-bit OS; 17.3&quot; screen 1920 x 1080; 60 Hz refreshing rate</td></tr><tr><td>Noise-blocking headphones</td><td>Bose</td><td>QC25</td><td>Headphones QuietComfort 25, noise-blocking</td></tr><tr><td>Decibel meter</td><td>Extech Instruments</td><td>SL 130G</td><td>Sound Level meter (dB), range 30 to 130 dB, this meter meets ANSI and IEC Type 2 sound level meter standards</td></tr><tr><td>&lt;strong&gt;Name&lt;/strong&gt;</td><td>&lt;strong&gt;Company&lt;/strong&gt;</td><td>&lt;strong&gt;Catalog Number&lt;/strong&gt;</td><td>&lt;strong&gt;Comments&lt;/strong&gt;</td></tr><tr><td>&lt;strong&gt;Software&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Labview</td><td>National Instruments</td><td>Labview 2014</td><td>Labview SP1 130, 64-bits, version 14</td></tr><tr><td>Matlab</td><td>Mathworks Inc</td><td>Matlab 2016a</td><td>The Mathworks Inc., Natick, MA, USA</td></tr><tr><td>GUI To create Visual and Acoustic stimuli. Created by Fabiola Duarte</td><td>Mathworks Inc</td><td>Matlab 2016a</td><td>The Mathworks Inc., Natick, MA, USA</td></tr></tbody></table>

a two-interval forced-choice task for multisensory comparisons

We provide a procedure for a psychophysics experiment in humans based on a previously described paradigm aimed to characterize the perceptual duration of intervals within the range of milliseconds of visual, acoustic, and audiovisual aperiodic trains of six pulses. In this task, each of the trials consists of two consecutive intramodal intervals where the participants press the upward arrow key to report that the second stimulus lasted longer than the reference, or the downward arrow key to indicate otherwise. The analysis of the behavior results in psychometric functions of the probability of estimating the comparison stimulus to be longer than the reference, as a function of the comparison intervals. In conclusion, we advance a way of implementing standard programming software to create visual, acoustic, and audiovisual stimuli, and to generate a two-interval forced-choice (2IFC) task by delivering stimuli through noise-blocking headphones and a computer&#39;s monitor.

This protocol presented a method to perform a psychophysics experiment in humans. The technique replicated previous research on the discrimination of intervals of AP trains of V, A, and AV pulses, which was performed using a 2IFC method. The stimuli resulted from P and AP distributions of trains of six 50-ms pulses in different intervals within the range of milliseconds (i.e., from 500 ms to 1,100 ms in steps of 100 ms). Figure 2A shows some interval...

Watch this Scientific Journal Video about A Two-interval Forced-choice Task for Multisensory Comparisons at JoVE.com

A Two-interval Forced-choice Task for Multisensory Comparisons

Psychophysics is essential for studying perception phenomena through sensory information. Here we present a protocol to perform a two-interval forced-choice task as implemented in a previous report on human psychophysics where participants estimated the duration of visual, auditory, or audiovisual intervals of aperiodic trains of pulses.

We provide a procedure for a psychophysics experiment in humans based on a previously described paradigm aimed to characterize the perceptual duration of ...

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Research

JoVE Journal

Behavior

10.9K Views.  Universidad Nacional Autónoma de México. Deze methode kan helpen bij het beantwoorden van kritische vragen op het gebied van perceptie neurowetenschappen. Zoals hoe tijdsintervallen worden geschat door verschillende zintuiglijke modaliteiten. Het belangrijkste voordeel van deze techniek is dat het metingen van nauwkeurigheid en reactietijden biedt die perceptuele resultaten van gedragsprocessen zijn.Hoewel deze methode inzicht kan geven in de menselijke waarneming, kan deze ook worden toegepast op andere diermodellen, zoals g...

Video: A Two-interval Forced-choice Task for Multisensory Comparisons

Perceptual and Category Processing of the Uncanny Valley Hypothesis' Dimension of Human Likeness: Some Methodological Issues

Mori's Uncanny Valley Hypothesis1,2 proposes that the perception of humanlike characters such as robots and, by extension, avatars (computer-generated characters) can evoke negative or positive affect (valence) depending on the object's degree of visual and behavioral realism along a dimension of human likeness (DHL) (Figure 1). But studies of affective valence of subjective responses to variously realistic non-human characters have produced inconsistent findings 3, 4, 5, 6. One of a number of reasons for this is that human likeness is not perceived as the hypothesis assumes. While the DHL can be defined following Mori's description as a smooth linear change in the degree of physical humanlike similarity, subjective perception of objects along the DHL can be understood in terms of the psychological effects of categorical perception (CP) 7. Further behavioral and neuroimaging investigations of category processing and CP along the DHL and of the potential influence of the dimension's underlying category structure on affective experience are needed. This protocol therefore focuses on the DHL and allows examination of CP. Based on the protocol presented in the video as an example, issues surrounding the methodology in the protocol and the use in "uncanny" research of stimuli drawn from morph continua to represent the DHL are discussed in the article that accompanies the video. The use of neuroimaging and morph stimuli to represent the DHL in order to disentangle brain regions neurally responsive to physical human-like similarity from those responsive to category change and category processing is briefly illustrated.

Investigation of the Uncanny Valley Hypothesis and affective experience requires an understanding of the hypothesis' dimension of human likeness (DHL). This protocol allows representation of the DHL and examination of categorical perception. Use of the same stimuli and fMRI to distinguish brain regions responsive to physical and category change is illustrated.

Mori's Uncanny Valley Hypothesis1,2 proposes that the perception of humanlike characters such as robots and, by extension, avatars (computer-generated ...

The Crossmodal Congruency Task as a Means to Obtain an Objective Behavioral Measure in the Rubber Hand Illusion Paradigm

The rubber hand illusion (RHI) is a popular experimental paradigm. Participants view touch on an artificial rubber hand while the participants' own hidden hand is touched. If the viewed and felt touches are given at the same time then this is sufficient to induce the compelling experience that the rubber hand is one's own hand. The RHI can be used to investigate exactly how the brain constructs distinct body representations for one's own body. Such representations are crucial for successful interactions with the external world. To obtain a subjective measure of the RHI, researchers typically ask participants to rate statements such as "I felt as if the rubber hand were my hand". Here we demonstrate how the crossmodal congruency task can be used to obtain an objective behavioral measure within this paradigm.
The variant of the crossmodal congruency task we employ involves the presentation of tactile targets and visual distractors. Targets and distractors are spatially congruent (i.e. same finger) on some trials and incongruent (i.e. different finger) on others. The difference in performance between incongruent and congruent trials - the crossmodal congruency effect (CCE) - indexes multisensory interactions. Importantly, the CCE is modulated both by viewing a hand as well as the synchrony of viewed and felt touch which are both crucial factors for the RHI.
The use of the crossmodal congruency task within the RHI paradigm has several advantages. It is a simple behavioral measure which can be repeated many times and which can be obtained during the illusion while participants view the artificial hand. Furthermore, this measure is not susceptible to observer and experimenter biases. The combination of the RHI paradigm with the crossmodal congruency task allows in particular for the investigation of multisensory processes which are critical for modulations of body representations as in the RHI.

We demonstrate how an objective measure can be employed in the widely employed rubber hand illusion paradigm. This measure is obtained by modifying the well-established crossmodal congruency task. This task allows the investigation of multisensory processes which are critical for modulations of body representations as in the rubber hand illusion.

The rubber hand illusion (RHI) is a popular experimental paradigm. Participants view touch on an artificial rubber hand while the participants' own hidden ...

Testing Sensory and Multisensory Function in Children with Autism Spectrum Disorder

In addition to impairments in social communication and the presence of restricted interests and repetitive behaviors, deficits in sensory processing are now recognized as a core symptom in autism spectrum disorder (ASD). Our ability to perceive and interact with the external world is rooted in sensory processing. For example, listening to a conversation entails processing the auditory cues coming from the speaker (speech content, prosody, syntax) as well as the associated visual information (facial expressions, gestures). Collectively, the &#8220;integration&#8221; of these multisensory (i.e., combined audiovisual) pieces of information results in better comprehension. Such multisensory integration has been shown to be strongly dependent upon the temporal relationship of the paired stimuli. Thus, stimuli that occur in close temporal proximity are highly likely to result in behavioral and perceptual benefits &#8211; gains believed to be reflective of the perceptual system&#39;s judgment of the likelihood that these two stimuli came from the same source. Changes in this temporal integration are expected to strongly alter perceptual processes, and are likely to diminish the ability to accurately perceive and interact with our world. Here, a battery of tasks designed to characterize various aspects of sensory and multisensory temporal processing in children with ASD is described. In addition to its utility in autism, this battery has great potential for characterizing changes in sensory function in other clinical populations, as well as being used to examine changes in these processes across the lifespan.

We describe how to implement a battery of behavioral tasks to examine the processing and integration of sensory stimuli in children with ASD. The goal is to characterize individual differences in temporal processing of simple auditory and visual stimuli and relate these to higher order perceptual skills like speech perception.

In addition to impairments in social communication and the presence of restricted interests and repetitive behaviors, deficits in sensory processing are ...

An Operant Intra-/Extra-dimensional Set-shift Task for Mice

Alterations in executive control and cognitive flexibility, such as attentional set-shifting abilities, are core features of several neuropsychiatric diseases. The most widely used neuropsychological tests for the evaluation of attentional set-shifting in human subjects are the Wisconsin Card Sorting Test (WCST) and the CANTAB Intra-/Extra-dimensional set shift task (ID/ED). These tasks have proven clinical relevance and have been modified and successfully adapted for research in animal models. However, currently available tasks for rodents present several limitations, mainly due to their manual-based testing procedures, which are hampering translational advances in psychiatric medicine. To overcome these limitations and to better mimic the original version in primates, we present the development of a novel operant-based two-chamber ID/ED "Operon" task for rodents. We demonstrated the effectiveness of this novel task to measure different facets of cognitive flexibility in mice including attentional set formation and shifting, and reversal learning. Moreover, we show the high flexibility of this task in which three different perceptual dimensions can be manipulated with a high number of stimuli cues for each dimension. This novel ID/ED Operon task can be an effective preclinical tool for drug testing and/or large genetic screening relevant to the study of executive dysfunction and cognitive symptoms found in psychiatric disorders.

Attentional set-shifting cognitive abnormalities are a core feature of many psychiatric diseases. Here, we present the development of a novel automatic system for the effective study of attentional set-shifting abilities, executive functions and other cognitive abilities in mice with high translational validity to human studies.

Alterations in executive control and cognitive flexibility, such as attentional set-shifting abilities, are core features of several neuropsychiatric ...

Cross-Modal Multivariate Pattern Analysis

Multivariate pattern analysis (MVPA) is an increasingly popular method of analyzing functional magnetic resonance imaging (fMRI) data1-4. Typically, the method is used to identify a subject's perceptual experience from neural activity in certain regions of the brain. For instance, it has been employed to predict the orientation of visual gratings a subject perceives from activity in early visual cortices5 or, analogously, the content of speech from activity in early auditory cortices6.
Here, we present an extension of the classical MVPA paradigm, according to which perceptual stimuli are not predicted within, but across sensory systems. Specifically, the method we describe addresses the question of whether stimuli that evoke memory associations in modalities other than the one through which they are presented induce content-specific activity patterns in the sensory cortices of those other modalities. For instance, seeing a muted video clip of a glass vase shattering on the ground automatically triggers in most observers an auditory image of the associated sound; is the experience of this image in the "mind's ear" correlated with a specific neural activity pattern in early auditory cortices? Furthermore, is this activity pattern distinct from the pattern that could be observed if the subject were, instead, watching a video clip of a howling dog?
In two previous studies7,8, we were able to predict sound- and touch-implying video clips based on neural activity in early auditory and somatosensory cortices, respectively. Our results are in line with a neuroarchitectural framework proposed by Damasio9,10, according to which the experience of mental images that are based on memories - such as hearing the shattering sound of a vase in the "mind's ear" upon seeing the corresponding video clip - is supported by the re-construction of content-specific neural activity patterns in early sensory cortices.

Classical multivariate pattern analysis predicts sensory stimuli a subject perceives from neural activity in the corresponding cortices (e.g. visual stimuli from activity in visual cortex). Here, we apply pattern analysis cross-modally and show that sound- and touch-implying visual stimuli can be predicted from activity in auditory and somatosensory cortices, respectively.

Multivariate pattern analysis (MVPA) is an increasingly popular method of analyzing functional magnetic resonance imaging (fMRI) data1-4. Typically, the ...

Neuroscience

Applying Incongruent Visual-Tactile Stimuli during Object Transfer with Vibro-Tactile Feedback

The application of incongruent sensory signals that involves disrupted tactile feedback is rarely explored, specifically with the presence of vibrotactile feedback (VTF). This protocol aims to test the effect of VTF on the response to incongruent visual-tactile stimuli. The tactile feedback is acquired by grasping a block and moving it across a partition. The visual feedback is a real-time virtual presentation of the moving block, acquired using a motion capture system. The congruent feedback is the reliable presentation of the movement of the block, so that the subject feels that the block is grasped and see it move along with the path of the hand. The incongruent feedback appears as the movement of the block diverts from the actual movement path, so that it seems to drop from the hand when it is actually still held by the subject, thereby contradicting the tactile feedback. Twenty subjects (age 30.2 &#177; 16.3) repeated 16 block transfers, while their hand was hidden. These were repeated with VTF and without VTF (total of 32 block transfers). Incongruent stimuli were presented randomly twice within the 16 repetitions in each condition (with and without VTF). Each subject was asked to rate the difficulty level of performing the task with and without the VTF. There were no statistically significant differences in the length of the hand paths and durations between transfers recorded with congruent and incongruent visual-tactile signals &#8211; with and without the VTF. The perceived difficulty level of performing the task with the VTF significantly correlated with the normalized path length of the block with VTF (r = 0.675, p = 0.002). This setup is used to quantify the additive or reductive value of VTF during motor function that involves incongruent visual-tactile stimuli. Possible applications are prosthetics design, smart sport-wear, or any other garments that incorporate VTF.

We present a protocol to apply incongruent visual-tactile stimuli during an object transfer task. Specifically, during block transfers, performed while the hand is hidden, a virtual presentation of the block shows random occurrences of false block drops. The protocol also describes adding vibrotactile feedback while performing the motor task.

The application of incongruent sensory signals that involves disrupted tactile feedback is rarely explored, specifically with the presence of vibrotactile ...

Using the Race Model Inequality to Quantify Behavioral Multisensory Integration Effects

Multisensory integration research investigates how the brain processes simultaneous sensory information. Research on animals (mainly cats and primates) and humans reveal that intact multisensory integration is crucial for functioning in the real world, including both cognitive and physical activities. Much of the research conducted over the past several decades documents multisensory integration effects using diverse psychophysical, electrophysiological, and neuroimaging techniques. While its presence has been reported, the methods used to determine the magnitude of multisensory integration effects varies and typically faces much criticism. In what follows, limitations of previous behavioral studies are outlined and a step-by-step tutorial for calculating the magnitude of multisensory integration effects using robust probability models is provided.

The current study aims to provide a step-by-step tutorial for calculating the magnitude of multisensory integration effects in an effort to facilitate the production of translational research studies across diverse clinical populations.

Multisensory integration research investigates how the brain processes simultaneous sensory information. Research on animals (mainly cats and primates) ...

Investigating Oculomotor Responses in Visual Duration Perception Tasks

Eye Movements in Visual Duration Perception: Disentangling Stimulus from Time in Predecisional Processes

Eye-tracking methods may allow the online monitoring of cognitive processing during visual duration perception tasks, where participants are asked to estimate, discriminate, or compare time intervals defined by visual events like flashing circles. However, and to our knowledge, attempts to validate this possibility have remained inconclusive so far, and results remain focused on offline behavioral decisions made after stimulus appearance. This paper presents an eye-tracking protocol for exploring the cognitive processes preceding behavioral responses in an interval comparison task, where participants viewed two consecutive intervals and had to decide whether it speeded up (first interval longer than second) or slowed down (second interval longer). 
Our main concern was disentangling oculomotor responses to the visual stimulus itself from correlates of duration related to judgments. To achieve this, we defined three consecutive time windows based on critical events: baseline onset, the onset of the first interval, the onset of the second interval, and the end of the stimulus. We then extracted traditional oculomotor measures for each (number of fixations, pupil size) and focused on time-window-related changes to separate the responses to the visual stimulus from those related to interval comparison per se. As we show in the illustrative results, eye-tracking data showed significant differences that were consistent with behavioral results, raising hypotheses on the mechanisms engaged. This protocol is embryonic and will require many improvements, but it represents an important step forward in the current state of art.

Author Spotlight: Exploring the Link Between Time Perception of Visual Stimuli and Reading Skills

We present a protocol that employs eye tracking to monitor eye movements during an interval comparison (duration perception) task based on visual events. The aim is to provide a preliminary guide to separate oculomotor responses to duration perception tasks (comparison or discrimination of time intervals) from responses to the stimulus itself.

Eye-tracking methods may allow the online monitoring of cognitive processing during visual duration perception tasks, where participants are asked to ...

A Psychophysics Paradigm for the Collection and Analysis of Similarity Judgments

Similarity judgments are commonly used to study mental representations and their neural correlates. This approach has been used to characterize perceptual spaces in many domains: colors, objects, images, words, and sounds. Ideally, one might want to compare estimates of perceived similarity between all pairs of stimuli, but this is often impractical. For example, if one asks a subject to compare the similarity of two items with the similarity of two other items, the number of comparisons grows with the fourth power of the stimulus set size. An alternative strategy is to ask a subject to rate similarities of isolated pairs, e.g., on a Likert scale. This is much more efficient (the number of ratings grows quadratically with set size rather than quartically), but these ratings tend to be unstable and have limited resolution, and the approach also assumes that there are no context effects.
Here, a novel ranking paradigm for efficient collection of similarity judgments is presented, along with an analysis pipeline (software provided) that tests whether Euclidean distance models account for the data. Typical trials consist of eight stimuli around a central reference stimulus: the subject ranks stimuli in order of their similarity to the reference. By judicious selection of combinations of stimuli used in each trial, the approach has internal controls for consistency and context effects. The approach was validated for stimuli drawn from Euclidean spaces of up to five dimensions. 
The approach is illustrated with an experiment measuring similarities among 37 words. Each trial yields the results of 28 pairwise comparisons of the form, "Was A more similar to the reference than B was to the reference?" While directly comparing all pairs of pairs of stimuli would have required 221445 trials, this design enables reconstruction of the perceptual space from 5994 such comparisons obtained from 222 trials.

The protocol presents an experimental psychophysics paradigm to obtain large quantities of similarity judgments, and an accompanying analysis workflow. The paradigm probes context effects and enables modeling of similarity data in terms of Euclidean spaces of at least five dimensions.

Similarity judgments are commonly used to study mental representations and their neural correlates. This approach has been used to characterize perceptual ...

Gaze-Contingent Framework for Studying and Training in Simulated Vision Loss

A Gaze-Contingent Display Framework for Perceptual Learning Research with Simulated Central Vision Loss

Macular degeneration (MD) is one of the leading causes of vision impairment in the Western world. Patients with MD tend to develop spontaneous eye movement strategies to compensate for their vision loss, including adopting a preferred retinal locus, or PRL, a spared peripheral region that they use more frequently to replace the damaged fovea. However, not all patients are successful in developing a PRL, and even when they do, it might take them months. Currently, no gold standard rehabilitative therapy exists, and MD research is further hindered by issues of recruitment, compliance, and comorbidity. To help address these issues, a growing body of research has used eye tracking-guided, gaze-contingent displays in a simulated central vision loss paradigm in individuals with intact vision. While simulated vision loss is qualitatively different than pathological central vision loss, our framework provides for a highly controlled model through which to study compensatory eye movements and test possible rehabilitation interventions in low vision. By developing a comprehensive framework, rather than relying on isolated and disconnected tasks, we create a cohesive environment where we can test larger-scale hypotheses, allowing us to examine interactions between tasks, assess training effects across multiple measures, and establish a consistent methodology for future research. Furthermore, participants in simulated central vision loss studies show similarities in their oculomotor compensatory behaviors compared to patients with MD. Here, we present a framework for conducting gaze-contingent studies related to simulated central vision loss. We emphasize the utilization of the framework to test behavioral and oculomotor performance of healthy individuals on a wide range of perceptual tasks encompassing different levels of visual processing. We also discuss how this framework can be adapted for training MD patients.

Development of a Gaze-Contingent Display Framework Designed for Perceptual and Oculomotor Research with Simulated Central Vision Loss

We present development of a gaze-contingent display framework designed for perceptual and oculomotor research simulating central vision loss. This framework is particularly adaptable for studying compensatory behavioral and oculomotor strategies in individuals experiencing both simulated and pathological central vision loss.

A Two-interval Forced-choice Task for Multisensory Comparisons

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