Selim Onat

I am a neuroscientist working currently on how humans make generalizations based on what they have previously learnt. To do so, I am using a variety of methodologies including fMRI (1), autonomous (2), as well as eye-movement recordings (3).

This research emanates from the well-established field of "stimulus generalization" following mainly the "lineage" of Hovland, Hull and Roger Shepard (4), and including the more recent computational work of Josua Tenenbaum (5). Furthermore, it integrates work on anxiety disorders, as it is believed that these mechanisms are impaired in people suffering from anxiety problems.

In the past, I have been working on how the nervous system processes natural scenes both at the electrophysiological and sensory-motor level. Since the times of Hubel and Wiesel, visual processing had been
overwhelmingly studied with artificial stimuli such as moving edges. However this type of stimuli suffer from an ecological validity problem, as they only rarely occur in real-life. We therefore investigated cortical processing during viewing of natural movies. This previous work focused on visual processing using mostly the technique of voltage-sensitive dye imaging and eye-tracking.

Guide for reproducing our BioRxiv Manuscript.

In this post, I would like to provide guidelines on how to reproduce results and figures presented in our recent manuscript in BioRxiv. I will show how to download the raw data, run the analysis routines,  and produce figures that are shown in the paper. The text below is also present in the section of the associated Matlab toolbox that is published as an open-source in GitHub.


With this repository you can reproduce all the analysis and figures presented in our paper:
Fear Generalization as Threat Prediction: Adaptive Changes in Facial Exploration Strategies revealed by Fixation-Pattern Similarity Analysis. Bioarxiv
In this paper, we investigate how humans explore faces and focus how these exploration patterns change when faces are associated with an aversive outcome.

Initial Setup

You can download the data and the associated repositories, including this one from the Open Science Framework here. Add these repositories to your Matlab path.


Get the list of participants.

The data you downloaded from the OSF contains all recorded participants. However, few had to be excluded for the main analysis. It is important to include all participants as it provide the possibility to reproduce the selection criteria used in the report or test the results with another selection criteria. 'get_subjects' action returns all the selected participants, totalling to 74.
>> FPSA_FearGen('get_subjects')
ans =
  Columns 1 through 45
     1     2     3     4     5     6     7     8     9    10    11    12    14    15    16    17    18    19    20    21    22    23    24    25    26    27    28    29    30    31    32    33    35    36    37    39    40    41    42    43    44    45    46    47    48
  Columns 46 through 74
    49    50    51    53    54    56    57    59    60    61    62    63    64    65    66    67    68    69    70    71    72    73    74    77    78    79    80    81    82

Sanity check 1.

In eye tracking it is usual that few trials are excluded due to blinks or bad calibration. The following code plots the number of trials per condition across the participant pool. The figure shows that almost all the particpants have their trials recorded as expected. This sanity check also shows that few participants have less trials than others.
4 above refers to the generalization phase. (2: baseline; 3: conditioning; 4: test phase).

Get the data to workspace using the get_fixmat action.

Fixmat is a compact way of storing large amounts of eye movement recordings in the form of fixation points.
>> fixmat = FPSA_FearGen('get_fixmat')
ans = 
  Fixmat with properties:

     subject: [1×118188 uint32]
       phase: [1×118188 int32]
       start: [1×118188 int32]
        stop: [1×118188 int32]
           x: [1×118188 single]
           y: [1×118188 single]
         eye: [1×118188 uint8]
    deltacsp: [1×118188 int32]
        file: [1×118188 int32]
     oddball: [1×118188 int32]
     trialid: [1×118188 int32]
         ucs: [1×118188 int32]
         fix: [1×118188 int32]
       chain: [1×118188 double]
       isref: [1×118188 double]
It stores every fixation's attributes in the form of separate vectors. For example, information such as x and y coordinates, participant's index, the image, the condition are stored in the Fixmat.
However, Fixmat variable above is not a simple Matlab structure though, but rather an instance of a Fixmat object, as coded in the FancyCarp Toolbox. The benefit of having a Fixmat object is that there are useful methods built-in in the Fixmat object, such as for example visualizing fixation density maps as heatmaps.
For example, the following code can be used to plot a fixation density map (FDM) based on all subjects (stored in S) during the generalization phase:
S = FPSA_FearGen('get_subjects');
fixmat = FPSA_FearGen('get_fixmat');
v{1} = {'subject' S 'phase' 4};
The Fixmat.getmaps method creates an FDM based on the cell array argument v. In the example above,v is used to select all fixations that belong to both phase 4 and subjects S. The method Fixmat.plot plots the computed FDM. In order to create a separate FDM for different conditions or participants we would create a separate cell array for each of the required filtering conditions.
for ns = S([11 3 8 23]);
  v{c} = {'subject' ns 'deltacsp' 0 'phase' 4};
This will plot a separate FDM for the 4 different participants.
Note how different participants scan faces differently. This is the basis for recent reports that analyzed the scan-path idiosyncrasy during viewing of faces. And the major reason for us to come up with the FPSA as a methodology to investigate how eye movement strategies change with aversive learning during viewing of faces.

Get FDM for the Fixation-Pattern Similarity Analysis

FPSA analysis is conducted on single-participants at a time. The get_fixmap action can be used to gather the required FDMs. It is basically a wrapper around the Fixmat.getmaps method.
>> maps = FPSA_FearGen('get_fixmap',fixmat,{'subject' 2});;
>> size(maps)
ans =
      250000          16
As you can see in maps each FDM is represented as a column vector. As we have 2 phases and 8 different conditions (faces), this amounts to 16 different vectors. This representation is appropriate for the similarity analysis as it can be readily used as an argument to Matlab's pdist

Fixation-Pattern Similarity Analysis

The following command will run a correlation-based similarity analysis on the FDMs of single participants, before (phase = 2) and after (phase =4) aversive learning. {'fix',1:100} indicates that the analysis will include 1st to 100th fixations, that is all the fixations.
1:3 ensures that similarity analysis is ran separately for the 3 runs of the generalization phase (phase = 4). This is important because the baseline phase (phase = 2) entails only one single run. In order to have a valid comparison of the similarity values across the baseline and test phases, the FDMs should contain on average same number of fixations. Computing separate FPSA matrices for each run ensures this.
sim = FPSA_FearGen('get_fpsa_fair',{'fix',1:100},1:3);
>> sim
sim = 
  struct with fields:

    correlation: [74×120 double]
The resulting matrix sim.correlation contains in each row the similarity data from a given participant. The pair-wise similarity values between the 16 FDMs are stored across the columns in a non-redundant format. You can visualize the resulting 16x16 matrix after putting the similarity values back to their positive-definite matrix format with the squareform function. 'plot_fpsa' action is used to do this.
The first quadrant of this matrix shows the similarity relationships between the 8 fixation-density maps recorded during the baseline period, before aversive learning has taken place. The second quadrant shows the same after learning has taken place.

Multi-dimensional Similarity Analysis

Another extremely useful way of representing the similarity relationships between the fixation maps consists of using the multi-dimensional scaling method. Below, I ran an MDS analysis using two dimensions.
fixmat = FPSA_FearGen('get_mdscale',squareform(mean(sim.correlation)),2)
This places a set of dots in such a way that the distances between each dot corresponds to the dissimilarity between the FDMs. Therefore, it is nice visual tool that summarizes the complex disssimilarity matrice, which is sometimes hard to digest. 
With the following piece of code, the upper part of the figure 03 of the manuscript can be produced summing up bundling previous figures.
FPSA_FearGen('figure_03A',FPSA_FearGen('get_fpsa_fair',{'fix' 1:100},1:3))

Analysis of dissimilarity matrices

One major aim in our paper was to understand how the similarity relationships between the FDMs changed with aversive learning. We take a linear modelling approach:
Y = bX
where Y is a non-redundant dissimilarity matrix, X is one of the 3 models we are testing, b are the coefficients that are to be fitted.
>> FPSA_FearGen('FPSA_get_table',{})
ans = 
      FPSA_B        FPSA_G         circle        specific      unspecific     Gaussian    subject    phase
    __________    ___________    ___________    ___________    ___________    ________    _______    _____
      -0.52828      -0.014812        0.70711     4.3298e-17        0.70711    0.48254     1          1    
     -0.064526        -0.3752    -1.0147e-17           -0.5            0.5    0.32924     1          1    
      -0.21919        -0.3426       -0.70711       -0.70711     8.6596e-17    0.25385     1          1    
      -0.28268       -0.20612             -1           -0.5           -0.5    0.32924     1          1    
      -0.35528      -0.052057       -0.70711    -1.2989e-16       -0.70711    0.48254     1          1    
       0.10652     -0.0068816    -1.7938e-16            0.5           -0.5    0.50102     1          1    
       0.27702        0.12585        0.70711        0.70711    -1.7319e-16    0.49959     1          1    
      -0.05106       -0.13439        0.70711    -4.3298e-17        0.70711    0.56373     1          1    
      -0.10373      -0.067047     6.1232e-17    -6.1232e-17     1.2246e-16      0.523     1          1    
       -0.1175       -0.39994       -0.70711    -4.3298e-17       -0.70711    0.56373     1          1  
The first two columns are the observed dissimilarity values for the first subject in the first phase (see last columns). Columns with names circlespecificunspecific and Gaussian are the modelled dissimilarity values that will be fitted for each participant separately.
In Matlab's statistical toolbox one can fit a linear model (pretty much the same way as in R) as follows:
fitlm(T,'FPSA_B ~ 1 + circle');
To to this for all the participants and models separately, we can run
T = FPSA_FearGen('FPSA_get_table',{});
[A,C] = FPSA_FearGen('FPSA_model_singlesubject',T)  
>> A.model_01
ans = 
  struct with fields:

    w1: [74×2 double]
>> A.model_01
ans = 
  struct with fields:

    w1: [74×2 double]
>> A.model_02
ans = 
  struct with fields:

    w1: [74×2 double]
    w2: [74×2 double]
The results of this analysis can be plotted with the following code, reproducing bottom panels of the figure 03.
FPSA_FearGen('figure_03C',{'fix' 1:100})