Research on the physiological mechanisms of 'recognition'

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Many studies have been done to investigate neural mechanisms of object recognition. In the visual modality, the flow of information is traced from the retina to the temporal association cortex, and it has been shown that the receptive fields of the neurons become larger and their trigger features become more comples, the later the stage of informaion processing. For example, neurons are reported in the superior temporal sulcus of the monkey which respond to faces. However, whatever complex trigger features these neurons in the superior temporal sulcus have, they code only 'what it is'. On the other hand, organisms extract the other aspect of the stimulus as well; 'meaning of the stimulus'. This study was conducted to find out neuronal substrates of object recognition in relation to the meaning of the stimulus, especially the 'associative significance of the stimulus'.Single unit activity was recorded from the frontal cortex of the monkey which was trained on a stimulus-reward association learning. The results indicated that about 28% of task-related units were related to this associative significance of the stimulus independent of its physical properties.Meaning-related properties were further examined by employing both visual and auditory stimuli which had the same associative significance. It was shown that the proportion of crossmodal meaning-related units which code the associative significance of the stimulus independent of its modality was relatively small among the cue-related differential units. Majority of frontal units behaved in different ways depending on the modality of the stimulus. The results indicate that the convergence of several modalities of sensory inputs in the frontal cortex does not well substantiate the crossmodal coding of the meaning of the stimulus.