“School of Cognitive Sciences”
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| Paper IPM / Cognitive Sciences / 11544 |
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The biased competition model (BCM) and the feature similarity gain model(FSGM) are two descriptive hypotheses of the mechanism and consequences of attentional influences on the responses of sensory neurons in striate and extrastriate visual cortex. The BCM assumes that visual stimuli compete for neuronal responses and attention biases the response of these neurons toward the response evoked by attended stimulus alone. The FSGM instead assumes that the sensory gain of cortical neurons is modulated as a function of the similarity between an attended feature and a given neuron?s preferred feature. For many experimental designs the two models give similar predictions, making it difficult to accumulate evidence for one or the other model. Here we designed an experiment in which the two models predict two qualitatively different results. We placed two moving random dot patterns (RDPs) inside the receptive field (RF) of neurons from area MT of macaque monkeys: one ?effective? (high contrast, evoking higher responses) and one ?ineffective? (low contrast, evoking lower responses). Both stimuli moved in the preferred direction of the cell, i.e. they did not differ in their feature-similarity according to the FSGM. Because the two RDPs evoke different responses when presented alone, the BCM predicts a lower response when the animals attended to the low contrast RDP. The FSGM on the other hand predicts the same response in both conditions, because the two RDPs move in the same direction and therefore the similarity between the attended and preferred feature does not change when attention is directed to one vs. the other RDP in the RF. We recorded from 81 well-isolated direction-selective cells in area MT of two macaque monkeys. A sub-population of 39 of these cells showed competition, i.e. a response to the two stimuli presented together that fell in between the responses evoked by each of the two stimuli alone. For this group of neurons (as well as for the whole population) no significant difference in responses was observed between attending to the effective vs. ineffective stimulus. The results are consistent with the prediction of the feature similarity gain model but not the biased competition model of attentional modulation. Consequently the data support an attentional mechanism that modulates the sensory gain of neurons based on the attentional condition rather than a mechanism that influences competitive interactions between distinct neuronal populations representing attended and unattended stimuli.
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