The Receptive Fields of Inferior Temporal Cortex Neurons in Natural Scenes

this paper indicate that the coordinates of the object in space that is to be the target for action are passed to the motor system by virtue of the facts that the object represented in the inferior temporal cortex in complex scenes is at the fovea and that the dorsal visual system that executes the actions has information about eye gaze position (cf. Ballard, 1991; Rolls and Deco, 2002

Responses of neurones of the rat suprachiasmatic nucleus to retinal illumination under photopic and scotopic conditions

We have examined the responses of neurones in the suprachiasmatic nuclei (SCN) of the rat to retinal illumination under photopic and scotopic conditions to identify the types of photoreceptor input to these nuclei. The majority of visually responsive SCN neurones studied under dark adaptation received rod input (48 of 52, 92 ). The action spectrum conformed to the sensitivity of rhodopsin, with maximal sensitivity at around 505 nm. When also studied under light adaptation, most visually responsive SCN neurones (20 out of 26, 77 ) responded to input from cones. The action spectra conformed to the spectrum of green cone opsin, with a main sensitivity peak at 510 nm and a significant secondary peak in the near-ultraviolet region of the spectrum. The frequency of spontaneous activity was typically low under scotopic conditions (range 0.2–17.2 Hz) and higher under photopic conditions (range 0.6–40 Hz) for any given neurone. The most common response under scotopic conditions was an ‘on-excitation’ (32 of 48, 62.5 ), which changed under photopic conditions to an on-excitation followed by a more prominent off-inhibition. Responses also changed due to endogenous ultradian cycles. Depending on the phase, responses could be altogether absent and even reverted from excitation to inhibition on opposite phases of a cycle. Ultradian cycles had a circadian dependence and were most common at around the light phase:dark phase (L:D) and D:L transition points of the circadian cycle. Under photopic conditions, SCN neurones showed rhythmic electrical activity, with a preferred firing interval that had a value between 18 and 39 ms. This rhythmic activity was probably the result of endogenous subthreshold membrane potential oscillations. In conclusion, light acting either via rod or cone pathways could have powerful, opposing actions on SCN neurones. These actions were state dependent. The presence of these neuronal responses suggests a role for rod and cone photoreceptors in SCN function

An Information Theoretic Approach to the Contributions of the Firing Rates

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The actions of monoamines and distribution of noradrenergic and serotoninergic contacts on different subpopulations of commissural interneurons in the cat spinal cord

Modulatory actions of monoamines were investigated on spinal commissural interneurons which coordinate left-right hindlimb muscle activity through direct projections to the contralateral motor nuclei. Commissural interneurons located in Rexed lamina VIM, with identified projections to the contralateral gastrocnemius-soleus motor nuclei, were investigated in deeply anaesthetized cats. Most interneurons had dominant input from either the reticular formation or from group II muscle afferents; a small proportion of neurons had input from both. Actions of ionophoretically applied serotonin and noradrenaline were examined on extracellularly recorded spikes evoked monosynaptically by group II muscle afferents or reticulospinal tract fibres. Activation by reticulospinal fibres was facilitated by both serotonin and noradrenaline. Activation by group II afferents was also facilitated by serotonin but was strongly depressed by noradrenaline. To investigate the possible morphological substrates of this differential modulation, seven representative commissural interneurons were labelled intracellularly with tetramethylrhodamine-dextran and neurobiotin. Contacts from noradrenergic and serotoninergic fibres were revealed by immunohistochemistry and analysed with confocal microscopy. There were no major differences in the numbers and distributions of contacts among the interneurons studied. The findings suggest that differences in modulatory actions of monoamines, and subsequent changes in the recruitment of subpopulations of commissural interneurons in various behavioural situations, depend on intrinsic interneuron properties rather than on the patterns of innervation by monoaminergic fibres. The different actions of noradrenaline on different populations of interneurons might permit reconfiguration of the actions of the commissural neurons according to behavioural context