A comparison of lateral and medial intraparietal areas during a visual categorization task

Abstract

Categorization is essential for interpreting sensory stimuli and guiding our actions. Recent studies have revealed robust neuronal category representations in the lateral intraparietal area (LIP). Here, we examine the specialization of LIP for categorization and the roles of other parietal areas by comparing LIP and the medial intraparietal area (MIP) during a visual categorization task. MIP is involved in goal-directed arm movements and visuomotor coordination but has not been implicated in non-motor cognitive functions, such as categorization. As expected, we found strong category encoding in LIP. Interestingly, we also observed category signals in MIP. However, category signals were stronger and appeared with a shorter latency in LIP than MIP. In this task, monkeys indicated whether a test stimulus was a category match to a previous sample with a manual response. Test-period activity in LIP showed category encoding and distinguished between matches and non-matches. In contrast, MIP primarily reflected the match/non-match status of test stimuli, with a strong preference for matches (which required a motor response). This suggests that, although category representations are distributed across parietal cortex, LIP and MIP play distinct roles: LIP appears more involved in the categorization process itself, whereas MIP is more closely tied to decision-related motor actions.

Figure 1.

Task design and behavioral performance. A, Monkeys were trained to categorize six directions of motion into one of two categories (denoted by the blue and red arrows). The boundary separating the two categories is shown by the dashed black line. B, Monkeys reported the category of the motion stimulus within a DMC task, releasing a touch bar if the sample and test stimuli belonged to the same category. If the category of the first test stimulus did not match the sample, a second test stimulus was shown (following a 150 ms delay) that was always a category match. The RF of the neuron is indicated by the dashed circle. C, Bar plot showing the percentage of trials for which the monkeys reported that the sample stimulus belonged to category 1 averaged across all LIP sessions. Performance was >90% for all conditions. Error bars indicate SE. D, Same as C, except for all MIP sessions.

Figure 2.

Average normalized response of LIP and MIP neurons. A, The neural activity averaged across all category-selective LIP (black curve) and MIP (gray curve) neurons, smoothed with a causal, 200 ms boxcar filter, is shown for match trials. The response is normalized so that the mean activity during the fixation period is one. Shaded area indicates SE. B, Same as A, except calculated using non-match trials.

Figure 3.

Example responses of LIP and MIP neurons. A–C, The average neural responses of three LIP neurons are shown. The response to the three motion directions from category 1 are shown in blue, and the response to the three motion directions from category 2 are shown in red. The subpanels to the right of each plot show the activity of the neurons for match (solid traces) and non-match (dotted traces) trials. D–F, Same as A–C, except showing the response of three MIP neurons.

Figure 4.

Strength of sample and test category selectivity. A, The mean sample category rCTI is shown for both LIP (black bars) and MIP (gray bars) for the fixation, sample, delay, and test epochs. The average is taken across all cells that were category selective for either the sample or test stimuli (see Materials and Methods). B, The mean test category rCTI is shown for the test epoch. Bars on the left show the mean across all cells that were category selective for the sample or test stimuli, whereas the bars to the right are the mean across all cells selective for just the test stimulus. Error bars indicate SE.

Figure 5.

Time course of category and direction selectivity. A, Diagram depicting how we classified the motion direction independent of any category selectivity. As an example, to classify the motion direction from the neural response to motion direction of 15° (dashed black arrow), the classifier was trained using trials with motion directions 15°, 255°, and 315° (blue arrows), which all belong to category 1. B, Diagram depicting how we classified the stimulus category independent of any direction selectivity. As an example, to classify the category from the neural response to motion direction of 15° (dashed black arrow), the classifier was trained using trials with motion directions 225° and 315° from the same category (blue arrows) and trials with motion directions 75° and 135° from the opposite category (red arrow). C, The accuracy in classifying the sample motion direction for LIP (black curve) and MIP (gray curve) as a function of time relative to sample onset. Black and gray horizontal bars indicate when decoding accuracy is significantly (p < 0.01) above chance according to a bootstrap analysis. Chance accuracy is 0.33. D, The accuracy in classifying the sample category direction for LIP (black curve) and MIP (gray curve) as a function of time relative to sample onset. Horizontal black and gray bars indicate when decoding accuracy is significantly above chance (p < 0.05, bootstrap). Chance accuracy is 50%. E, The cumulative latency distributions for direction selectivity to develop in LIP (dashed black curve) and category selectivity to develop in LIP (solid back curve) and MIP (solid gray curve). Because direction selectivity in MIP was weak, we could not obtain an accurate latency distribution. F, Same as D, except for the test category.

Figure 6.

Relating neural activity to the visual stimulus and the manual bar release. A, Normalized neural activity averaged across LIP (left column) and MIP (right column) neurons relative to the test onset. For each neuron, trials were divided based on whether their RT was among the fastest third (blue curve), middle third (red curve), or slowest third (green curve). The colored asterisks indicate the average RT for each trial set. B, Same as A, except neural activity relative to the manual bar release is shown. C, The trial-by-trial correlation coefficient between the spike count in the preceding 200 ms with the RT of the trial averaged across LIP (left column) and MIP (right column) neurons. D, Histogram showing the normalized difference between the coefficients linking neural activity to the time since test onset and the time until bar release, for LIP (left column) and MIP (right column) neurons.

Figure 7.

Anatomical location and neuronal activity during delayed memory saccade task. A, MRI scan from one monkey showing the location of the two cortical areas, LIP and MIP, targeted in this study. Area LIP is located on the lateral bank of the IPS, whereas area MIP is located on the medial bank of the IPS. B, The average neural response during the delayed memory saccade task for all LIP (black) and MIP (gray) neurons in response to the saccade target in the preferred direction of the neuron (i.e., the direction that elicited the greatest spike rate). Error bars denote one SE. C, Histogram showing saccade direction tuning index for all LIP (black) and MIP (gray) neurons.