Arnold L. Towe
Gary W. Harding
C. Fred Tyner
Judy K. Nyquist
Microelectrodes are biased toward isolating larger neurons as opposed to smaller ones. To determine the magnitude of this bias across cortical depth, the pyramidal-tract latency distribution of the antidromically activated M neurons in the post-Cruciate site (1) was determined. This was compared to the sample expected when the fiber spectrum of the pyramidal tract was converted to a time of arrival distribution based upon fiber diameter and to the known distribution of cell sizes in this tissue. A significant under sampling in layers II and lower III (dominated by small pyramidal and stellate neurons) was detected (Towe and Harding, 1970)
Laminar recordings (100 micron steps over cortical depth) of the extracellular field potentials were recorded in the post-Cruciate site (1) and the first derivative over depth calculated to derive current flow. The inverse of interspike interval serves as a proxy for the slope and magnitude of the underlying EPSP/IPSP that produced the discharge pattern and therefore, the extracellular current flow associated with it. The single neuron sample at this site was used to simulate the extracellular current flow produced by each neural response at the depth the neuron was isolated. These current flows were summed across depth to determine the expected volume current flow. Contour plots of this and the directly derived pattern were found to be a very close match (Towe, 1966) even though the microelectrode sampling bias was not included.
Figure. Modulation of CF latency (Observed/Expected Latency). Observed (solid lines) and expected (dashed line) mean first-spike latencies to 1/sec surpramaximal stimulation of each of the four paws. Vertical hatching shows region of no modulation.
The IF, CH, and IH first spike latencies for M neurons in the post-Cruciate site (1) were used to determine where the CF response would be based purely on conduction distance. For each neuron, the off-focus Z-score was determined to produce the expected on-focus distribution. Then the ratio of observed CFo latency divided by expected CFe latency (O/E) for each neuron was calculated. The data were then grouped by this ratio and sample mean CF, IF, CH, and IH latencies calculated for each group. These results were plotted with the O/E ratio on the X axis and latency on the Y axis. This showed that there was an inverse relation between the off-focus and on-focus responses. This was interpreted as modulation of the on-focus response by off-focus inputs (Towe et al., 1976). Not included in this report was a subset of Sa neurons which responded very early to CF stimulation and not at all to IF, CH, and IH stimulation (extending the on-focus CFo response just to the left of the graph). These Sa neurons were suspected of actually being M neurons with a subthreshold off-focus input. However, this turned out not to be the case (Satterthwaite et al., 1978). They turned out to be a subset of the Sa class. In addition, there was a subset of neurons which responded very early to strong IF, CH, and IH stimulation and did not respond to CF stimulation (extending the off-focus IF, CH, and IH responses just to the right of the graph). These neurons completely inhibited the CFo on-focus response and were called I neurons (Tyner et al., 1977). Curiously, the effect was not apparent at weak stimulus intensities.
REFERENCES
Satterthwaite, W. R., J. A. Burnham, and A. L. Towe. 1978. Wide-field conditioning effects on small-field neurons in the posterior sigmoid gyrus of domestic cats. Exp. Neurol. 60:603-613.
Towe, A. L. 1966. On the nature of the primary evoked response. Exp. Neurol. 15:113-139.
Towe, A. L. and G. W. Harding. 1970. Extracellular microelectrode sampling bias. Exp. Neurol. 29:366-381.
Towe, A. L., C. F. Tyner, and J. K. Nyquist. 1976. Facilitatory an inhibitory modulation of wide-field neuron activity in postcruciate cerebral cortex of the domestic cat. Exp. Neurol. 50:734-756.
Tyner, C. F. and M. G. Miller. 1977. Selective inhibition of some wide-field sensorimotor cortex neurons by high intensity skin stimuli. Neurosci. Abs. 3:72.