Responses of Neurons in Inferior Temporal Cortex During Memory-Guided Visual Search

Chelazzi, Leonardo, John Duncan, Earl K. Miller, and Robert Desimone. Responses of neurons in inferior temporal cortex during memory-guided visual search. J. Neurophysiol. 80: 2918–2940, 1998. A typical scene will contain many different objects, few of which are relevant to behavior at any given moment. Thus attentional mechanisms are needed to select relevant objects for visual processing and control over behavior. We examined this role of attention in the inferior temporal cortex of macaque monkeys, using a visual search paradigm. While the monkey maintained fixation, a cue stimulus was presented at the center of gaze, followed by a blank delay period. After the delay, an array of two to five choice stimuli was presented extrafoveally, and the monkey was rewarded for detecting a target stimulus matching the cue. The behavioral response was a saccadic eye movement to the target in one version of the task and a lever release in another. The array was composed of one “good” stimulus (effective in driving the cell when presented alone) and one or more “poor” stimuli (ineffective in driving the cell when presented alone). Most cells showed higher delay activity after a good stimulus used as the cue than after a poor stimulus. The baseline activity of cells was also higher preceding a good cue, if the animal expected it to occur. This activity may depend on a top-down bias in favor of cells coding the relevant stimulus. When the choice array was presented, most cells showed suppressive interactions between the stimuli as well as strong attention effects. When the choice array was presented in the contralateral visual field, most cells initially responded the same, regardless of which stimulus was the target. However, within 150–200 ms of array onset, responses were determined by the target stimulus. If the target was the good stimulus, the response to the array became equal to the response to the good stimulus presented alone. If the target was a poor stimulus, the response approached the response to that stimulus presented alone. Thus the influence of the nontarget stimulus was eliminated. These effects occurred well in advance of the behavioral response. When the array was positioned with stimuli on opposite sides of the vertical meridian, the contralateral stimulus appeared to dominate the response, and this dominant effect could not be overcome by attention. Overall, the results support a “biased competition” model of attention, according to which 1) objects in the visual field compete for representation in the cortex, and 2) this competition is biased in favor of the behaviorally relevant object by virtue of “top-down” feedback from structures involved in working memory.

Effect of Similarity Training on Imitation

The effect of increasing the reward value of stimulus similarity in a match-to-sample task upon subsequent imitation (behavioral similarity) was assessed. Subjects ( N = 48) were assigned to three groups: an experimental group (similarity) who received reinforcement for stimulus matching, and two control groups who served to help assess the effects of reinforcement and exposure to the match-to-sample task. Subjects were then exposed to televised models demonstrating task-oriented and task-incidental behaviors, and their subsequent imitation was observed. Only the results for males with task-incidental imitation were consistent with the hypothesis, and several research and clinical implications were discussed.

Stimulus Encoding and Additivity of Effects in a Card Sorting Task

The work of Sternberg (1969) suggests the independence from one another of the times required for encoding a stimulus, matching it against a set of stored representations in memory and responding. A card sorting experiment was performed to test some implications of this position. The variables manipulated were the number of different letters to be sorted into a pile, orientation of the letter on a card, and whether letters encodable according to a shared physical characteristic were to be sorted into the same (within) or different (between) piles. As predicted, orientation of the letter and number of different letters to be sorted/pile affected sorting time only in the between pile condition where the effects were observed to be additive.

Effect of Presenting Matching Stimuli on the Orienting Response

72 Ss were assigned to 4 experimental and 6 control groups in a study of the effects of 2 different levels of probability of occurrence of a matching stimulus and a standard stimulus upon the orienting response (OR) to those stimuli. In addition, the dimension of similarity of standard and matching stimuli was assessed at 2 levels. Using skin conductance as the measure of the OR, results indicated that generalization of habituation occurred. There was no evidence that the dimension of similarity of standard stimulus-matching stimulus affected the degree of generalization. It was concluded that theoretical models of the OR be cautious in assigning precise generalization of habituation a central position.