Larry A. Palmer

Flexible, foldable, actively multiplexed, high-density electrode array for mapping brain activity in vivo

Arrays of electrodes for recording and stimulating the brain are used throughout clinical medicine and basic neuroscience research, yet are unable to sample large areas of the brain while maintaining high spatial resolution because of the need to individually wire each passive sensor at the electrode-tissue interface. To overcome this constraint, we developed new devices that integrate ultrathin and flexible silicon nanomembrane transistors into the electrode array, enabling new dense arrays of thousands of amplified and multiplexed sensors that are connected using fewer wires. We used this system to record spatial properties of cat brain activity in vivo, including sleep spindles, single-trial visual evoked responses and electrographic seizures. We found that seizures may manifest as recurrent spiral waves that propagate in the neocortex. The developments reported here herald a new generation of diagnostic and therapeutic brain-machine interface devices.

Visual receptive fields of single striate cortical units projecting to the superior colliculus in the cat

Summary Single units in the striate cortex of anesthetized, paralyzed cats were activated antidromically by electrical stimulation of the superior colliculus and their receptive fields were plotted and studied. These corticotectal neurons were found to lie in layer V and, based on standard criteria 21,22,30 , the majority were complex cells. Most of the corticotectal neurons were driven about equally by the two eyes and none were strictly monocular. All of the corticotectal neurons were orientation selective, a large proportion were direction selective and they tended to have large receptive fields. The corticotectal units differed from most other cortical cells in that they lacked any clear summation with stimulus length and, in general, responded very well to small moving spots. The data reinforces the idea, based on ablation studies 4,32,42 that direction selectivity and the effectiveness of the ipsilateral eye in driving collicular units are dependent on binocular, direction selective inputs from the striate cortex. The data were also discussed from the point of view of information sorting within the visual cortex and its efferent projections.

An autoradiographic study of the projections of the dorsal lateral geniculate nucleus and the posterior nucleus in the cat

Injections of small quantities of tritiated amino acid were made in the dorsal lateral geniculate nucleus (LGNd) and the adjacent posterior nucleus (PN), and the cortical projections of these nuclei were studied using the autoradiographic tracing method. It was concluded that the laminar division of LGNd projects only to areas 17 and 18 of the ipsilateral hemisphere. That portion of LGNd designated the medial interlaminar nucleus (MIN) projects to areas 18, 19 and the Clare—Bishop are (CB). PN was found to project to areas 19, CB, and a region within the splenial sulcus. No subcortical projections of LGNd or PN were observed. The distribution of transported label within cortical laminae showed that terminations occur largely in layer IV of all cortical areas studied, and that a relatively small number of terminations take place in layers I–III and in layer V. The cortical labeling pattern in areas 17 and 18 indicated that a substantial number of terminations may also exist in layer VI. The initial trajectory of PN and MIN axons was studied. PN fibers coursed laterally and remained closely applied to the dorsal border of LGNd before turning dorsally into the visual radiations. Most MIN fibers, on the other hand, passed laterally through the laminae of LGNd before ascending to the cortex.

Contribution of linear spatiotemporal receptive field structure to velocity selectivity of simple cells in area 17 of cat

We have examined the spatiotemporal structure of simple receptive fields in the cats striate cortex by cross-correlating their spike trains with an ensemble of stimuli consisting of stationary bright and dark spots whose position was randomized on each 50 msec frame. Receptive fields were found to be either separable or inseparable in space-time and responses to moving stimuli were predicted from the spatiotemporal structure of the cell under study. Most simple cells with separable spatiotemporal receptive fields were not direction selective. All simple cells with inseparable spatiotemporal receptive fields were found to prefer movement in one direction. The optimal speed and direction were estimable from the slope of individual subregions observed in the space-time plane. The results are consistent with a linear model for direction selectivity.

Visual receptive field properties of cells of the superior colliculus after cortical lesions in the cat

Abstract Single units were studied in the superficial gray and optic laminae of the superior colliculus of cats during light barbiturate anesthesia. Cells in normal animals reponded maximally to moving stimuli, with 75% showing direction selectivity and 80% being strongly driven by either eye (binocular convergence). Large chronic cortical lesions that included area 17, or lesions limited to area 17 alone, resulted in a much reduced proportion of cells in the ipsilateral colliculus showing direction selectivity (12%) and binocular convergence (17%). In contrast, after lesions no change was seen in the spatial properties of collicular receptive fields. Large cortical lesions sparing area 17 did not produce the above effects on collicular cells. Cortical lesions placed 1 hr to 3 days before recording produced the same loss in direction selectivity and binocular convergence as lesions of 16 months duration. We conclude that in the cat these properties of collicular cells are dependent upon the integrity of cortical area 17.

Contribution of linear mechanisms to the specification of local motion by simple cells in areas 17 and 18 of the cat

A reverse correlation technique, which permits estimation of three-dimensional first-order properties of receptive fields (RFs), was applied to simple cells in areas 17 and 18 of cat. Two classes of simple cells were found. For one class, the spatial and temporal RF characteristics were separable, i.e. they could be synthesized as the product of spatial and temporal weighting functions. RFs in the other class were inseparable, i.e. bright and dark subregions comprising each field were obliquely oriented in space-time. Based on a linear superposition model, these observations led to testable hypotheses: (1) simple cells with separable space-time characteristics should be speed but not direction selective and (2) simple cells with inseparable space-time characteristics should be direction selective and the optimal velocity of moving stimuli should be predictable from the slope of the oriented subregions. These hypotheses were tested by comparing responses to moving bars with those predicted by application of the convolution integral. Linear predictions accounted for waveforms of responses to moving bars in detail. For cells with oriented space-time characteristics, the preferred direction was always predicted correctly and the optimal speed was predicted quite well. Most cells with separable space-time characteristics were not direction selective as predicted. The major discrepancies between measured and predicted behavior were twofold. First, 8/32 cells with separable space-time RFs were direction selective. Second, predicted directional indices were weakly correlated with actual measurements. These conclusions hold for simple cells in both areas 17 and 18. The major difference between simple RFs in these areas is the coarser spatial scale seen in area 18. These results demonstrate a significant linear contribution to the speed and direction selectivity of simple cells in areas 17 and 18. Where additional, nonlinear mechanisms are inferred, they appear to act synergistically with the linear mechanism.

Stimulus Feature Selectivity in Excitatory and Inhibitory Neurons in Primary Visual Cortex

Although several lines of evidence suggest that stimulus selectivity in somatosensory and visual cortices is critically dependent on unselective inhibition, particularly in the thalamorecipient layer 4, no comprehensive comparison of the responses of excitatory and inhibitory cells has been conducted. Here, we recorded intracellularly from a large population of regular spiking (RS; presumed excitatory) and fast spiking (FS; presumed inhibitory) cells in layers 2–6 of primary visual cortex. In layer 4, where selectivity for orientation and spatial frequency first emerges, we found no untuned FS cells. Instead, the tuning of the spike output of layer 4 FS cells was significantly but moderately broader than that of RS cells. However, the tuning of the underlying synaptic responses was not different, indicating that the difference in spike-output selectivity resulted from differences in the transformation of synaptic input into firing rate. Layer 4 FS cells exhibited significantly lower input resistance and faster time constants than layer 4 RS cells, leading to larger and faster membrane potential (Vm) fluctuations. FS cell Vm fluctuations were more broadly tuned than those of RS cells and matched spike-output tuning, suggesting that the broader spike tuning of these cells was driven by visually evoked synaptic noise. These differences were not observed outside of layer 4. Thus, cell type-specific differences in stimulus feature selectivity at the first level of cortical sensory processing may arise as a result of distinct biophysical properties that determine the dynamics of synaptic integration.

Projections of the pulvinar-lateral posterior complex to visual cortical areas in the cat

Abstract The projections of the pulvinar-lateral posterior complex of the cat were studied using the autoradiographic tracing method and related to 15 previously defined cortical areas. The results indicate that each of three separate zones within the pulvinar-lateral posterior complex has a different pattern of projection. The most lateral zone, the pulvinar, sends fibers to at least seven cortical areas, most of which are known to have input from other visual areas within the brain: the splenial visual area, the cingulate gyrus, and areas 5, 7, 19, 20a and 21a. A zone located just medial to the pulvinar, the lateral division of the lateral posterior complex, projects to at least eight visual areas in the cortex: areas 17, 18, 19, 20a, 21a, 21b, the posteromedial lateral suprasylvian area and the ventral lateral suprasylvian area. The most medial zone, the intermediate division of the lateral posterior complex, projects to at least four cortical areas: 20a, the posterior suprasylvian area, the posterolateral lateral suprasylvian area and the dorsal lateral suprasylvian area. Of the 15 cortical areas that receive fibers from the pulvinar-lateral posterior complex, only three (areas 19, 20a and 21a) receive projections from more than one of these thalamic zones, and only one of the cortical areas (20a) receives fibers from all three zones. Thus, the data support the division of the pulvinar lateral posterior complex into three zones on the basis of their unique and largely non-overlapping projections to the visual cortex.

The two-dimensional spatial structure of nonlinear subunits in the receptive fields of complex cells

We have estimated the second-order response properties of complex cells in two spatial dimensions by cross-correlating their spike trains with a binary approximation of a Gaussian white noise stimulus ensemble. Wiener-like kernels were computed and generally consisted of two or three parallel, elongated subregions alternating between augmented and suppressed response. These subunits were scattered across the receptive fields of complex cells and their axes of elongation agreed with the optimal orientation determined with drifting gratings.

Temporal diversity in the lateral geniculate nucleus of cat.

Reverse correlation was used in conjunction with ternary white noise to estimate the first-order spatiotemporal receptive-field structure of LGN cells in the anesthetized, paralyzed cat. Based on a singular-value decomposition of these data, we conclude that most LGN cells are approximately space-time separable. An analysis of the timecourses of the first singular values revealed a strongly bimodal but continuous distribution of rise times and waveforms. The two modes represented cells generally associated with the lagged and nonlagged classes of Mastronarde (1987a,b), and this was confirmed by their responses to step and sine-modulated spots in their field centers. The intermediate cells, rather than appearing to constitute a separate group, smoothly filled the region between the obviously lagged and nonlagged cells in every respect. These conclusions are limited to X-cells although the data from a much smaller population of Y-cells conform to the same scheme. We conclude that lagged and nonlagged cells represent the modes of a continuous and very broad distribution of temporal responses in the cat LGN.