Cholinergic Control of Visual Categorization in Macaques

Acetylcholine (ACh) is a neurotransmitter acting via muscarinic and nicotinic receptors that is implicated in several cognitive functions and impairments, such as Alzheimer’s disease. It is believed to especially affect the acquisition of new information, which is particularly important when behavior needs to be adapted to new situations and to novel sensory events. Categorization, the process of assigning stimuli to a category, is a cognitive function that also involves information acquisition. The role of ACh on categorization has not been previously studied. We have examined the effects of scopolamine, an antagonist of muscarinic ACh receptors, on visual categorization in macaque monkeys using familiar and novel stimuli. When the peripheral effects of scopolamine on the parasympathetic nervous system were controlled for, categorization performance was disrupted following systemic injections of scopolamine. This impairment was observed only when the stimuli that needed to be categorized had not been seen before. In other words, the monkeys were not impaired by the central action of scopolamine in categorizing a set of familiar stimuli (stimuli which they had categorized successfully in previous sessions). Categorization performance also deteriorated as the stimulus became less salient by an increase in the level of visual noise. However, scopolamine did not cause additional performance disruptions for difficult categorization judgments at lower coherence levels. Scopolamine, therefore, specifically affects the assignment of new exemplars to established cognitive categories, presumably by impairing the processing of novel information. Since we did not find an effect of scopolamine in the categorization of familiar stimuli, scopolamine had no significant central action on other cognitive functions such as perception, attention, memory, or executive control within the context of our categorization task

Natural scene perception: Inferior temporal cortex neurons encode the positions of different objects in the scene

Inferior temporal cortex (IT) neurons have reduced receptive field sizes in complex natural scenes. This facilitates the read-out of information about individual objects from IT, but raises the question of whether more than the single object present at the fovea is represented by the firing of IT neurons, as would be important for whole scene perception in which several objects may be located without eye movements. Recordings from IT neurons with five simultaneously presented objects, each subtending 7°, with one object at the fovea and the other four centred 10 ° eccentrically in the parafovea, showed that although 38 IT neurons had their best response to an effective stimulus at the fovea, eight IT neurons had their best response to an object when it was located in one or more of the parafoveal positions. Moreover, of 54 neurons tested for asymmetric parafoveal receptive fields, 35 (65%) had significantly different responses for different parafoveal positions. The asymmetry was partly related to competition within the receptive fields, as only 21 % of the neurons had significant asymmetries when tested with just one object present located at the same parafoveal positions. The findings thus show that some evidence is conveyed by a population of IT neurons about the relative positions of several simultaneously presented objects in a scene extending well into the parafovea during a single fixation, and this is likely to be important in whole scene perception with multiple objects, including specifying the relative positions of different objects in a scene

Object perception in natural scenes: Encoding by inferior temporal cortex simultaneously recorded neurons

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The use of decoding to analyze the contribution to the information of the correlations between the firing of simultaneously recorded neurons

A new decoding method is described that enables the information that is encoded by simultaneously recorded neurons to be measured. The algorithm measures the information that is contained not only in the number of spikes from each neuron, but also in the cross-correlations between the neuronal firing including stimulus-dependent synchronization effects. The approach enables the effects of interactions between the ‘signal’ and ‘noise’ correlations to be identified and measured, as well as those from stimulus-dependent cross-correlations. The approach provides an estimate of the statistical significance of the stimulus-dependent synchronization information, as well as enabling its magnitude to be compared with the magnitude of the spike-count related information, and also whether these two contributions are additive or redundant. The algorithm operates even with limited numbers of trials. The algorithm is validated by simulation. It was then used to analyze neuronal data from the primate inferior temporal visual cortex. The main conclusions from experiments with two to four simultaneously recorded neurons were that almost all of the information was available in the spike counts of the neurons; that this Rate information included on average very little redundancy arising from stimulus-independent correlation effects; and that stimulus-dependent cross-correlation effects (i.e. stimulus-dependent synchronization) contribute very little to the encoding of information in the inferior temporal visual cortex about which object or face has been presented

Neuronal Correlates of Auditory Streaming in Monkey Auditory Cortex for Tone Sequences without Spectral Differences

This study finds a neuronal correlate of auditory perceptual streaming in the primary auditory cortex for sequences of tone complexes that have the same amplitude spectrum but a different phase spectrum. Our finding is based on microelectrode recordings of multiunit activity from 270 cortical sites in three awake macaque monkeys. The monkeys were presented with repeated sequences of a tone triplet that consisted of an A tone, a B tone, another A tone and then a pause. The A and B tones were composed of unresolved harmonics formed by adding the harmonics in cosine phase, in alternating phase, or in random phase. A previous psychophysical study on humans revealed that when the A and B tones are similar, humans integrate them into a single auditory stream; when the A and B tones are dissimilar, humans segregate them into separate auditory streams. We found that the similarity of neuronal rate responses to the triplets was highest when all A and B tones had cosine phase. Similarity was intermediate when the A tones had cosine phase and the B tones had alternating phase. Similarity was lowest when the A tones had cosine phase and the B tones had random phase. The present study corroborates and extends previous reports, showing similar correspondences between neuronal activity in the primary auditory cortex and auditory streaming of sound sequences. It also is consistent with Fishman's population separation model of auditory streaming

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Information in the first spike, the order of spikes, and the numbe

Exp Brain Res (2004) 155: 370--384

A new decoding method is described that enables the information that is encoded by simultaneously recorded neurons to be measured. The algorithm measures the information that is contained not only in the number of spikes from each neuron, but also in the cross-correlations between the neuronal firing including stimulus-dependent synchronization effects. The approach enables the effects of interactions between the `signal' and `noise' correlations to be identified and measured, as well as those from stimulus-dependent cross-correlations. The approach provides an estimate of the statistical significance of the stimulus-dependent synchronization information, as well as enabling its magnitude to be compared with the magnitude of the spike-count related information, and also whether these two contributions are additive or redundant. The algorithm operates even with limited numbers of trials. The algorithm is validated by simulation. It was then used to analyze neuronal data from the primate inferior temporal visual cortex. The main conclusions from experiments with two to four simultaneously recorded neurons were that almost all of the information was available in the spike counts of the neurons; that this Rate information included on average very little redundancy arising from stimulus-independent correlation effects; and that stimulus-dependent cross-correlation effects (i.e. stimulus-dependent synchronization) contribute very little to the encoding of information in the inferior temporal visual cortex about which object or face has been presented

Biol. Cybern. 90, 19–32 (2004) DOI 10.1007/s00422-003-0451-5

Information encoding in the inferior temporal visual cortex: contributions of the firing rates and the correlations between the firing of neuron

Neuronal Correlates of Auditory Streaming in Monkey Auditory Cortex for Tone Sequences without Spectral Differences

This study finds a neuronal correlate of auditory perceptual streaming in the primary auditory cortex for sequences of tone complexes that have the same amplitude spectrum but a different phase spectrum. Our finding is based on microelectrode recordings of multiunit activity from 270 cortical sites in three awake macaque monkeys. The monkeys were presented with repeated sequences of a tone triplet that consisted of an A tone, a B tone, another A tone and then a pause. The A and B tones were composed of unresolved harmonics formed by adding the harmonics in cosine phase, in alternating phase, or in random phase. A previous psychophysical study on humans revealed that when the A and B tones are similar, humans integrate them into a single auditory stream; when the A and B tones are dissimilar, humans segregate them into separate auditory streams. We found that the similarity of neuronal rate responses to the triplets was highest when all A and B tones had cosine phase. Similarity was intermediate when the A tones had cosine phase and the B tones had alternating phase. Similarity was lowest when the A tones had cosine phase and the B tones had random phase. The present study corroborates and extends previous reports, showing similar correspondences between neuronal activity in the primary auditory cortex and auditory streaming of sound sequences. It also is consistent with Fishman’s population separation model of auditory streaming