Effects of subcortical ablations on cortical association responses in the cat

Retinotopic organization within the cat's posterior suprasylvian sulcus and gyrus

The organization of extrastriate visual areas of the cat's posterior suprasylvian sulcus and gyrus was studied with electrophysiological mapping methods. Analysis of retinotopic organization confirmed the presence of dorsal lateral and ventral lateral suprasylvian (DLS, VLS) visual areas (Palmer et al., '78, J. Comp. Neurol. 177:237-256) and demonstrated new features of organization. Areas DLS and VLS occupy the upper two-thirds of the posterior suprasylvian sulcus, with DLS wholly confined to the upper bank and VLS straddling the sulcal fundus. Both areas contain a partial representation of the lower quadrant of the visual field. A narrow strip of visually responsive cortex (periauditory belt) was identified adjoining DLS on the posterior ectosylvian gyrus; its organization and extent were not explored in detail. The organization of the posterior suprasylvian areas (PS) (Updyke, '82, Soc. Neurosci. Abst. 8:810) was explored in detail. Area PS lies inferior to areas VLS and 21b in the lower third of the posterior suprasylvian sulcus and gyrus, extending onto the fusiform gyrus. PS contains a partial representation of the lower quadrant of the visual field. It shares a representation of horizontal meridian with area 21b and a representation of central gaze with area VLS. Analysis of the PS/20 border region indicates that the representation of the lower quadrant periphery is common to the borders of PS, 20a, and 20b rather than lying internal to areas 20a and 20b as suggested by Tusa and Palmer ('80, J. Comp. Neurol. 193:147-164).

Depth evoked potential and single unit correlates of vertex midlatency auditory evoked responses

The rostral brainstem and thalamus of awake cats were studied for depth correlates of a surface-recorded midlatency auditory evoked potential, wave "A', with a latency range of 17-25 ms. In response to clicks or pure tones, midlatency potentials were recorded from the level of the cuneiform nucleus (14-15 ms latencies) forward through the medial tegmentum to the level of the intralaminar thalamic nuclei centralis lateralis (CL) and center median (CM) (17-19 ms latencies). While this medial projection system to the thalamus involved primarily CL and CM, slightly longer latency responses were also found in nucleus ventralis anterior (VA) and ventralis lateralis (VL). A ventral diencephalic response was characterized by latencies averaging 0.5-1.2 ms less than those from the dorsal thalamic regions. Both surface and depth midlatency potentials showed comparable sensitivity to rate of stimulation. At click rates above 1/s, peak amplitudes diminished, and for rates greater than 10/s, both surface and depth midlatency responses were abolished. This rate sensitivity differs from that of the auditory brainstem responses (ABRs), which are essentially unchanged at rates of 20/s. Whereas ABRs are unaffected by surgical levels of sodium pentobarbital, the surface and depth midlatency potentials are replaced by a deep negativity within minutes following administration of anesthesia. Extracellular recordings of acoustically responsive single units in CL and CM demonstrated latencies comparable to the CL and CM field potential latencies. Both the units and field potentials were similarly rate sensitive. Each auditory unit showed a best frequency response, but none demonstrated somatosensory convergence. Bilateral aspiration of the inferior colliculus did not abolish the midlatency depth or surface responses. Rather, recordings in CL and CM suggested response enhancement over a two week postoperative period. Taken together these data suggest that the midlatency vertex potential, wave "A', reflects a generator system which projects from cuneiform nucleus, through the medial tegmentum to the medial thalamus, particularly to CL and CM. The functional significance of this medial auditory projection system remains to be determined. It could mediate physiological correlates of "state', modulate sensory input or motor output, or it could provide an integrative mechanism for the focusing of auditory attention.

Diencephalic projections from the pontine reticular formation: autoradiographic studies in the cat

Ascending projections to the diencephalon from the pontine reticular formation were studied in the cat by autoradiographic techniques. Projections from both rostral and caudal pontine regions ascend to the caudal diencephalon and divide into two components; a dorsal leaf terminates primarily in the thalamic intralaminar complex and a ventral leaf terminates in the subthalamic region. The relative densities of the two terminal regions vary with the injection site. Fibers originating in the caudal pons (nucleus reticularis pontis caudalis) terminate relatively heavily in the intralaminar nuclei of the dorsal thalamus, particularly the centre median, central lateral, central dorsal and paracentral nuclei, and also the dorsal medial nucleus. Relatively sparse termination occurs in the subthalamic region. In contrast, fibers from the rostral pons (nucleus reticularis pontis oralis) terminate relatively heavily in the subthalamic region, including the zona incerta, the fields of Forel, the ventral part of the thalamic reticular complex, and the lateral hypothalamus. Relatively sparse termination occurs in the dorsal thalamus, but includes the centre median, parafascicular, central lateral, paracentral and dorsal medial nuclei. These data are discussed with regard to reticular control of forebrain activity and the role of the classic dorsal and ventral components of ascending reticular projections.

Patterns of habituation to electrical stimulation of reticular and auditory pathways

Patterns of habituation of the orienting response to electrical stimulation of midbrain, thalamus, and cortical components of the primary auditory pathway and the ascending reticular system were studied in awake, freely mobile cats. Individual orienting responses to high-frequency stimulation of structures in the auditory pathway could not be distinguished from orienting responses to stimulation of reticular structures. However, with repeated presentations of the stimulus, the orienting response to auditory pathway stimulation habituated significantly faster than did the orienting response to stimulation of structures in the ascending reticular system. Within the auditory pathway, orienting to stimulation of the inferior colliculus, the medial geniculate body, or primary auditory cortex did not result in distinguishable patterns of habituation. Stimulation of parietal cortex resulted in habituation that occurred significantly slower than habituation to auditory cortex stimulation, and significantly faster than habituation to stimulation of the midbrain reticular formation or the centre median nucleus of the thalamus. These results are discussed with regard to a hypothetical arousal system that includes the brain stem reticular formation, the intralaminar nuclei of the thalamus, and parts of the parietal cortex.

Compound stimulus differentiation behavior in the rhesus monkey following periarcuate ablations

Anatomical and physiological investigations in cat and monkeys have demonstrated the existence of association areas where evoked and unit responses associated with more than one sensory modality can be recorded. Although it is often assumed that these areas of association cortex form the anatomical substratum for complex cognitive and integrative behavior in higher mammals, there is little experimental evidence in non-human primates to support this viewpoint. This experiment investigated the effects of periarcuate ablations on the ability of the rhesus monkey to perform a compound stimulus (light and tone) versus component stimuli (light or tone) differentiation. Ablation of this well-known polysensory area in the frontal lobe led to large and persistent impairments in performance on this task, while the ability to perform intramodal and intermodal differentiations was spared. Ablations of comparable size in other parts of the frontal lobe had no affect on the performance of these tasks. It was concluded that one function of the periarcuate region may be to synthesize multiple sources of sensory input occurring simultaneously into meaningful elements.

Thalamic projections to visually responsive regions of parietal cortex

Thalamic efferents to visually responsive regions of parietal cortex of the cat were investigated by experimental retrograde tracing techniques. Two classes of photically evoked potentials recorded from the middle suprasylvian gyrus could be distinguished on the basis of waveform and latency. Type 1 responses were recorded from the Clare-Bishop area at the lateral border of the suprasylvian gyrus and Type 2 responses were recorded from association response areas on the crown of the suprasylvian gyrus. Studies of retrograde degeneration in kittens and adult cats and retrograde transport of intracortically injected horseradish peroxidase indicate that afferents to the electrophysiologically identified Clare-Bishop area originate in the lateroposterior nucleus, the posterior nucleus, and the medial interlaminar nucleus of the lateral geniculate body. Afferents to electrophysiologically identified association response areas originate in the lateroposterior nucleus, ventroanterior nucleus, the pulvinar, the laterodorsal nucleus, and the central lateral nucleus. Studies of orthograde degeneration following placement of parietal lesions indicate a close reciprocity of corticothalamic and thalamocortical projections.