The spatial receptive field of thalamic inputs to single cortical simple cells revealed by the interaction of visual and electrical stimulation

Abstract

Electrical stimulation of the thalamus has been widely used to test for the existence of monosynaptic input to cortical neurons, typically with stimulation currents that evoke cortical spikes with high probability. We stimulated the lateral geniculate nucleus (LGN) of the thalamus and recorded monosynaptically evoked spikes from layer 4 neurons in visual cortex. We found that with moderate currents, cortical spikes were evoked with low to moderate probability and their occurrence was modulated by ongoing sensory (visual) input. Furthermore, when repeated at 8-12 Hz, electrical stimulation of the thalamic afferents caused such profound inhibition that cortical spiking activity was suppressed, aside from electrically evoked monosynaptic spikes. Visual input to layer 4 cortical cells between electrical stimuli must therefore have derived exclusively from LGN afferents. We used white-noise visual stimuli to make a 2D map of the receptive field of each cortical simple cell during repetitive electrical stimulation in the LGN. The receptive field of electrically evoked monosynaptic spikes (and thus of the thalamic input alone) was significantly elongated. Its primary subfield was comparable to that of the control receptive field, but secondary (flanking) subfields were weaker. These findings extend previous results from intracellular recordings, but also demonstrate the effectiveness of an extracellular method of measuring subthreshold afferent input to cortex.

Fig 1.

Presumed thalamocortical circuit and visual responses in normal and silenced cortex. (A Upper) Model of thalamocortical circuit. Circular receptive fields of thalamic cells are transformed to an elongated cortical receptive field by the convergence of inputs (adapted from refs. and 27). On subfields are shown in red, and off subfields are blue. In this model, recurrent intracortical connections (excitatory, from pyramidal and spiny stellate cells; and inhibitory) amplify thalamic input but are not required for generating the elongated cortical receptive field. (Lower) Schematic responses to a drifting sine grating stimulus, decomposed into a thalamic component (green) and a larger intracortical component (blue). At optimal phases of the visual stimulus the summed components (black) exceed threshold and induce spiking. (B Upper) Thalamocortical circuit when cortical spiking is silenced by feed-forward inhibition. (Lower) Schematic responses to visual stimulus during repetitive electrical stimulation of the LGN at a rate of 8–12 Hz (red). Electrical stimulation spares the monosynaptic visually evoked thalamic input but silences intracortical activity in the period between electrical stimuli. The result is that the cortical membrane potential remains below spiking threshold at all times except when an electrical pulse is delivered. The relative probability of electrically evoked spiking is modulated by the residual visual input from the LGN. Thus repetitive electrical stimulation serves double duty: it silences cortex and probes the visual drive from the LGN.

Fig 2.

Efficacy of electrical stimulation varies with stimulus current (A) and phase of sine grating visual stimulus (B). (A) Raster plot and PSTH from the responses of a simple cell to 200 repeats of a drifting grating visual stimulus under control conditions (0 μA) and during electrical stimulation of the LGN (5 and 50 μA). Electrical stimuli were applied close to the peak of the visual response (red arrow). Five-microampere stimulation led to a slightly higher probability (6%) of the cortical cell firing a spike within ≈2 ms, compared with control. This relatively weak stimulation current partially suppressed the ongoing visually evoked response. Fifty-microampere electrical stimulation of the same cell evoked monosynaptic spikes with high probability (42%) and almost completely silenced the remaining activity. PSTH bin width: 2.5 ms. (B) Response PSTH of a different cortical cell to a drifting sine grating stimulus (blue histogram, ordinate at left; bin width = 3 ms). Superimposed is the efficacy of electrical stimulation applied at different phases of the visual stimulus (red curve, ordinate at right). Electrical stimuli were applied every 90 ms, whereas the visual stimulus was drifted with a period of 240 ms. Thus, for every three cycles of the visual stimulus, the electrical stimulus occurred at eight distinct phases. Other than monosynaptic spikes, repetitive electrical stimulation silenced >98% of cortical activity. The efficacy of electrical stimulation in producing a cortical spike reflects the subthreshold visually evoked input from the thalamus. Error bars (± SD) were obtained by calculating the efficacy separately for 10 equal segments of the data.

Fig 3.

Cortical response to combined white noise and electrical stimulation. (A) Each frame of the white-noise visual stimulus was a unique checkerboard (16 × 16 matrix) of white and black squares. Four such frames, which were presented every 29.4 ms, are shown. Electrical stimuli (red arrows) were delivered at the start of every fourth frame (every fifth frame for some experiments). (B) Raster plot of simple-cell responses to combined electrical and white-noise visual stimulation. Electrical stimuli were applied at time 0, as shown in A. Each electrical stimulus (200 μA) coincided with the start of every fourth visual frame (total frames: 32,768); therefore, 8,192 electrical stimuli were presented over the course of ≈16 min. In this example, only 4 of 4,838 spikes occurred later than ≈2 ms after the electrical stimulus. (C) Raster plot of responses to white-noise visual stimulation alone. Without electrical stimulation, spikes (1,298 total) occurred at more evenly distributed times. In this example there were more spikes in the stimulated condition than in the control, but this was not always the case.

Fig 4.

Spatial receptive fields of cortical layer 4 simple cells and of their thalamic input. (A–D) Receptive fields of four simple cells under control conditions. Off subfields are shown in blue, and on subfields are shown in red. Gridlines show individual stimulus pixels (0.4–0.6°). The elongated primary subfield of each cortical cell was fitted with a 2D Gaussian (shown as ellipses at 1.75σ). (E–H) Receptive field of the thalamic input for the four cells shown in A–D, as obtained with combined visual and electrical stimulation. In all four cases, the primary subfield remained elongated. Stimulation currents for E, F, G, and H were 100, 200, 225, and 200 μA, respectively. (I–L) Primary subfield length vs. width for simple cells (black) and their thalamic input (red) for each of the four cases shown in A–H. Each point corresponds to parameters derived from one iteration of our error analysis (see Materials and Methods). All values lie above the diagonal line of unit slope, indicating that the receptive fields of the cortical neurons and their thalamic inputs were both significantly elongated (Wilcoxon signed rank tests, P < 0.0001 in all cases).

Fig 5.

Extent of electrically evoked silencing depends on the retinotopic separation of the LGN stimulation and cortical recording sites. The retinotopic position of each site was defined by taking the absolute value of the receptive field (single-unit in cortex, multiunit in LGN), thresholding, and then calculating the center of mass. The greater the receptive field separation, the more leakage or unsuppressed spikes detected in the 89-ms window preceding each electrical stimulus (as a fraction of the control condition). The two recordings with absolute suppression (zero leakage spikes) were plotted at the arbitrary level of 0.01% leakage and were not included in the regression.