By analogy to previous work on lateral geniculate nucleus (LGN) magnocellular (M) and parvocellular (P) cells our goal was to construct a physiological profile of koniocellular (K) cells that might be linked to particular visual perceptual attributes. Extracellular recordings were used to study LGN cells, or their axons, in silenced primary visual cortex (V1), in nine anaesthetized owl monkeys injected with a neuromuscular blocker. Receptive field centre‐surround organization was examined using flashing spots. Spatial and temporal tuning and contrast responses were examined using drifting sine‐wave gratings; counterphase sine‐wave gratings were used to examine linearity of spatial summation. Receptive fields of 133 LGN cells and 10 LGN afferent axons were analysed at eccentricities ranging from 2.8 to 31.3 deg. Thirty‐four per cent of K cells and only 9 % of P and 6 % of M cells responded poorly to drifting gratings. K, P and M cells showed increases in centre size with eccentricity, but K cells showed more scatter. All cells, except one M cell, showed linearity in spatial summation. At matched eccentricities, K cells exhibited lower spatial and intermediate temporal resolution compared with P and M cells. K contrast thresholds and gains were more similar to those of M than P cells. M cells showed lower spatial and higher temporal resolution and contrast gains than P cells. K cells in different K LGN layers differed in spatial, temporal and contrast characteristics, with K3 cells having higher spatial resolution and lower temporal resolution than K1/K2 cells. Taken together with previous results these findings suggest that the K cells consist of several classes, some of which could contribute to conventional aspects of spatial and temporal resolution.

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We examined 66 complex cells in area 17 of cats that were paralyzed and anesthetized with propofol and N2O. We studied changes in ensemble responses for small (<10 degrees ) and large (>10 degrees ) differences in orientation. Examination of temporal resolution and discharge history revealed advantages in discrimination from both dependent (e.g., synchronization) and independent (e.g., bursting) interspike interval properties. For 27 pairs of neurons, we found that the average cooperation (the advantage gained from the joint activity) was 57.6% for fine discrimination of orientation but <5% for gross discrimination. Dependency (probabilistic quantification of the interaction between the cells) was measured between 29 pairs of neurons while varying orientation. On average, the dependency tuning for orientation was 35.5% narrower than the average firing rate tuning. The changes in dependency around the peak orientation (at which the firing rate remains relatively constant) lead to substantial cooperation that can improve discrimination in this region. The narrow tuning of dependency and the cooperation provide evidence to support a population-encoding scheme that is based on biologically plausible mechanisms and that could account for hyperacuities.

There is no clear link between the broad tuning of single neurons and the fine behavioral capabilities of orientation discrimination. We recorded from populations of cells in the cat visual cortex (area 17) to examine whether the joint activity of cells can support finer discrimination than found in individual responses. Analysis of joint firing yields a substantial advantage (i.e., cooperation) in fineangle discrimination. This cooperation increases to more considerable levels as the population of an assembly is increased. The cooperation in a population of six cells provides encoding of orientation with an information advantage that is at least 2-fold in terms of requiring either fewer cells or less time than independent coding. This cooperation suggests that correlated or synchronized activity can increase information.T raditionally, research in sensory cortex has sought to link sensory information with changes in the impulse rate of single neurons (1), under the presumption that the rate code is the primary signaling modality. The principle of a rate code has also been applied to a population of cells (2, 3), but concerns still arise about the ambiguities of rates, and the ability of a rate code to account for perceptual phenomena such as association, generalization, hyperacuity, and contextual modulation has been debated (2-8). Alternative theories of cortical function (4-8) require a reliable means of preserving the temporal information of impulses in a cortical network of unreliable synapses. Information-theoretic methods demonstrate the existence of links between the temporal structure of neural responses and sensory input (9-11), and mechanisms capable of generating predictable temporal patterns (e.g., bursts and oscillations) by means of intrinsic cellular properties and network interactions have been well documented (12)(13)(14). However, temporal information is relevant only when an appropriate decoding mechanism exists (15). Synchronization between neural assemblies provides one such mechanism for encoding, and possibly decoding, temporal structure because synchrony is dependent on temporal patterns (17)(18)(19) and is correlated with the orientation (17-19) and coherence (17,18,20) of visual stimuli.We have previously shown (21) that the orientation selectivity of dependency (a numeric measure reflecting synchrony) between two cells is on average 35.5% narrower than the selectivity of their individual rate tuning. Similar results have been documented with other methods (19,22). We also measured positive synergy (i.e., cooperation; information available only in the joint activity of the cells) among pairs of cells, at least for discriminating fine differences in orientation (21). The average bin width (4.6 ms) and discharge history (total time of 9.2 ms) of this analysis suggested that the cooperation results from correlated (or synchronous) activity. Traditionally, correlation among cells has been considered a limitation in the information capacity of population coding (23). However, because correl...

Iontophoretic injections of horseradish peroxidase (HRP) into either the preoptic area or ventral hypothalamus of the green treefrog, (Hyla cinerea), demonstrated inputs from thalamic and midbrain auditory nuclei. In a pattern similar to that seen in Rana catesbeiana and Rana pipiens, the central thalamic and secondary isthmal nuclei were found to provide heavy input to the ventral hypothalamus. Additionally, a lighter input from the anterior thalamic nucleus was seen. In contrast, the preoptic area receives a major input from the anterior thalamic and secondary isthmal nuclei, and possibly a sparse input from the central thalamic nucleus. These results suggest that in treefrogs multimodal and auditory information may reach the preoptic area and ventral hypothalamus, two regions involved in endocrine regulation and the control of reproductive behavior, via largely separate major pathways from the thalamus combined with a common midbrain input. Furthermore, the ventral hypothalamus receives heavy input from the preoptic area, lateral amygdala, suprachiasmatic nucleus, anterior entopeduncular nucleus, and a lighter input from the striatum. Nonauditory afferents to the preoptic area originate in the medial and lateral septal nuclei, medial pallium, and the dorsal-, lateral-, and ventral hypothalamus. The preoptic area and ventral hypothalamus are reciprocally connected.

A sinusoidal mask grating oriented orthogonally to and superimposed onto an optimally oriented base grating reduces a cortical neuron's response amplitude. The spatial selectivity of cross-orientation suppression (XOR) has been described, so for this paper we investigated the temporal properties of XOR. We recorded from single striate cortical neurons (n = 72) in anesthetized and paralyzed cats. After quantifying the spatial and temporal characteristics of each cell's excitatory response to a base grating, we measured the temporal-frequency tuning of XOR by systematically varying the temporal frequency of a mask grating placed at a null orientation outside of the cell's excitatory orientation domain. The average preferred temporal frequency of the excitatory response of the neurons in our sample was 3.8 (± 1.5 S.D.) Hz. The average cutoff frequency for the sample was 16.3 (± 1.7) Hz. The average preferred temporal frequency (7.0 ± 2.6 Hz) and cutoff frequency (20.4 ± 6.9 Hz) of the XOR were significantly higher. The differences averaged 1.1 (± 0.6) octaves for the peaks and 0.3 (± 0.4) octaves for the cutoffs. The XOR mechanism's preference for high temporal frequencies suggests a possible extrastriate origin for the effect and could help explain the low-pass temporal-frequency response profile displayed by most striate cortical neurons.

Horseradish applications in the ventral hypothalamus of the northern leopard frog, Rana pipiens, demonstrated inputs from three auditory nuclei, the anterior and central thalamic nuclei and the secondary isthmal nucleus, in a pattern identical to that seen in Rana catesbeiana. Single-unit recordings from the area receiving these inputs revealed neurons with high spontaneous firing rates. In approximately half of the isolated neurons, the firing rate could be increased or, less often, decreased, by more than 20% by the presentation of a repetitive noise burst or conspecific mating call. Treatment with human chorionic gonadotropin prior to recording resulted in no significant differences in the number or response properties of the acoustically sensitive hypothalamic cells. These results suggest that acoustic signals such as those used for intraspecific communication can modulate neural activity in a part of the basal forebrain involved in endocrine-regulation.

Human psychophysical and animal behavioral studies have illustrated the benefits that can be conferred from having information available from multiple senses. Given the central role of multisensory integration for perceptual and cognitive function, it is important to design behavioral paradigms for animal models to provide mechanistic insights into the neural bases of these multisensory processes. Prior studies have focused on large mammals, yet the mouse offers a host of advantages, most importantly the wealth of available genetic manipulations relevant to human disease. To begin to employ this model species for multisensory research it is necessary to first establish and validate a robust behavioral assay for the mouse. Two common mouse strains (C57BL/6J and 129S6/SvEv) were first trained to respond to unisensory (visual and auditory) stimuli separately. Once trained, performance with paired audiovisual stimuli was then examined with a focus on response accuracy and behavioral gain. Stimulus durations varied from 50 ms to 1 s in order to modulate the effectiveness of the stimuli and to determine if the well-established “principle of inverse effectiveness” held in this model. Response accuracy in the multisensory condition was greater than for either unisensory condition for all stimulus durations, with significant gains observed at the 300 ms and 100 ms durations. Main effects of stimulus duration, stimulus modality and a significant interaction between these factors were observed. The greatest behavioral gain was seen for the 100 ms duration condition, with a trend observed that as the stimuli became less effective, larger behavioral gains were observed upon their pairing (i.e., inverse effectiveness). These results are the first to validate the mouse as a species that shows demonstrable behavioral facilitations under multisensory conditions and provides a platform for future mechanistically directed studies to examine the neural bases of multisensory integration.