Effects of Task Demands on the Responses of Color-Selective Neurons in the Inferior Temporal Cortex. Koida, Komatsu. NatNeuro 2007

  1. “Categorization and fine discrimination are two different functions in visual perception, and we can switch between these two functions depending on the situation or task demands. To explore how visual cortical neurons behave in such situations, we recorded the activities of color-selective neurons in the inferior temporal (IT) cortex of two monkeys trained to perform a color categorization task, a color discrimination task and a simple fixation task. Many IT neurons changed their activity depending upon the task, although color selectivity was well conserved. A majority of neurons showed stronger responses during the categorization task. Moreover, for the population of IT neurons as a whole, signals contributing to performing the categorization task were enhanced. These results imply that judgment of color category by color-selective IT neurons is
    facilitated during the categorization task and suppressed during the discrimination task as a consequence of task-dependent modulation of their activities.”
  2. “On the one hand, we are able to discriminate subtle differences in color; on the other hand, we often categorize similar colors into a single group, such as ‘red’ or ‘green’. How we utilize each of these two functions depends upon the demands of the situation or task: in some situations we respond similarly to colors A and B (categorization); in other situations we respond differently to the two colors (discrimination).”
    1. <Discrimination isn’t the opposite of categorization – it can be thought of categorization where two items are simply placed in different categories…>
    2. Task switching happens in PFC, and there are neurons there that encode task rules, and change responses as soon as tasks change
  3. Question is whether neuronal activity in sensory cortices changes with changes in task rules
  4. “Many studies have shown that attention to specific locations or visual features modulates
    the activities of visual cortical neurons. However, it is generally believed that stimulus coding and neuronal responses within visual cortical areas are stable with respect to changes in a task rule, though few studies have directly examined this problem.”
  5. Here they examine if color sensitive neurons in inferior temporal (IT) cortex change based on task.  In one case monkeys have to make a categorical judgement, and in the other a find discrimination to the same color stimulus
  6. “The IT cortex lies at the final stage of the ventral visual pathway15, and it has strong mutual connections with the PFC16. Single-unit recordings have shown that many color-selective neurons exist within the IT cortex…”
  7. Three tasks, all which present a colored stimulus that monkey fixated on:
    1. Categorization: classify color as closer to red or green, either by maintaining fixation or saccading
    2. Discrimination: choose which of two choice stimuli was the same color as the sample stimulus
    3. Passive viewing
    4. <Not yet clear how the discrimination worked, was it with a saccade to one of the two colors?  If so it seems it would have been better to have the categorization task also involve a saccade to one of two locations instead of either fixation or saccade>
  8. “We found that the activity of many IT neurons differed depending upon the task, although color selectivity was well conserved. For the population of IT neurons as a whole, color signals differentiating red versus green were enhanced during the categorization task. This suggests that judgment of color category by color-selective IT neurons was facilitated during the categorization task and suppressed during the discrimination task as a consequence of task dependent modulation of these neurons. These results suggest that the flow of color information from the IT cortex is strongly controlled by top-down signals representing the ongoing task rule presumably sent from the PFC.”

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