DISCUSIÓN DE RESULTADOS DE LA INVESTIGACIÓN
VALORES MÁXIMOS ADMISISBLES MODIFICADO CON D.S.001-VIVIENDA CAPÍTULO
e f f e c t iv e n e ss. Clear bars
(tops only plotted) = the Best category, hatched bars = the Mid category and solid bars correspond to the Worst category of responses. Firing rate is expressed as a percentage of pealc response in Best category. Lo w e r: St a t ist ic a l e v a l u a t io n OF r e spo n se d isc r im in a t io n. Probability levels of 0.05, 0.005 and 0.001 shown. Discrimination between stimuli first reaches significance {P < 0.05) for the cell population at 116 ms after stimulus onset.
Vogels and Orbaii (1991) showed that the variability of neural responses in primate VI increased with time lapsed from response onset. Similarly the variability in firing rate of STPa cells assessed with small time bins (thin line) appears to increase with time. This the increase is not apparent in the initial stages of the response but develops slowly over hundreds of milliseconds. Thus any eai'ly discrimination seen within 100 ms of response onset is not due to low response variability allowing statistical detection of a small relative difference in firing rates. During the early part of response (first 100 ms), firing rate is high, discrimination is clear (Figure 3.6) and variability is low (Figure 3.4). During the late part of response, firing rate is lower, discrimination is poor and variability is high.
Population response
For the 22 cells with a 5.2 ms time bin found to produce a response in each of the three response categories the amplitude normalized responses (S/A = 0%, Pealc from Best = 100%) were averaged by response category to produce the population PSTH response profiles (Figure 3.5a). The parameters as measured for the population responses are shown in Table 3.3a. The increased rise time and 1/2 fall time compared with the mean of the individual cells can be explained in part by the differing response latencies and in part by the reduced variability of firing rate (assessed on a 5 ms basis) when responses are averaged over cells.
A 2-way ANOVA (fixed factor = response category, random factor = the 22 cells) was performed between the three categories of response each of the 250 time bins. The resulting F ratio values, where the rank ordering of the responses was Best > Mid > Worst, are plotted against time in Figure 3.5b. The discrimination between stimuli achieves statistically very reliable and stable levels at 116 ms after stimulus onset. The latency of response of this cell population (n=22) occurs 90 ms after stimulus onset.
Efficiency of the ventral pathway (3.67) Table 3.3
Parameter Values of the Population and Average Cell Responses
(a) Population (b) Average Cell
Parameter Best Mid Worst Best Mid Worst
Timing (ms) Latency 95.2 90.0 105 . 6 100.4 100.4 105 . 6 Rise Time 62.4 36.4 31.2 57.2 31.2 36.4 1/2 fall Time 119.6 166.4 46.8 130.0 140. 4 52.0 Decay Time 842.4 603.2 57.2 842.4 306.8 57.2 Duration 904.8 639.6 88.4 899.6 338.0 93.6 Firing Rate (%)'* Peak 95.6 56.4 32.4 89.8 53.3 32.7 1st 100ms 69.8 37.6 20.5 72.5 41.8 21.2 2nd 100ms 58.1 36.8 10.9 53.7 33.4 1 0 .6 5th 100ms 23.9 14.4 4.9 24.7 13.4 4.7 Last 100ms 22.8 12 . 5 4.6 23.5 11.7 5 . 6
Normalized to Peak Response Magnitudes ( % )
1st 100ms 73.0 66.7 63.3 80.7 78.4 64.8
2nd 10 0ms 60.8 65.2 33.6 59.8 62.7 32.4
5th 100ms 25.0 25.5 15 .1 27.5 25.1 14 . 4
Last 100ms 23.8 22.1 14.2 26.2 22.0 17.1
Values of response parameters for 3 levels of stimulus effectiveness are listed for (a) the cell population and (b) the Average Cell after synchronisataion of response onsets. '"The normalization of firing rates set S/A = 0 and peak for each cell to 100. The values are expressed as percentage of the maximum possible firing rate (100).
I
I
Average Cell Response n=22
Threshold S/A
0 500 ,
POST-STIMULUS TIME (ms)
Average Cell Discrimination
S
I
I
0 500 POST-STIMULUS TIME (ms) p 0.001 p 0.005 p 0.05 Av e r a g e Ce l l RESPONSE AND DISCRIMINATION. Up p e r: Av e r a g e MAGNITUDE OF RESPONSES FOR 22 CELLS TO THREE CATEGORIES OF STIMULUS EFFECTIVENESS. The response latencies of contributing cells were synchronized to 100 ms (post-stimulus). Best, Mid, Worst categories denoted by clear, hatched and solid traces respectively. Lo w e r: St a t ist ic a l e v a l u a t io n OF r e spo n se DISCRIMINATION BETWEEN THE 3 RESPONSE CATEGORIES (F ratio computed for each time bin). Discrimination between stimuli reaches significance (P < 0.05) within 5 ms of response onset.Efficiency of the ventral pathway (3.68)
A more accurate estimate of the population latency was obtained by examining the responses of all cells selective for head views (n=44). The latency of this ku'ger cell population was 80 ms, with discrimination between Best and Worst occurring at 95 ms.
Average Cell response
Figure 3.6a shows the average response from the 22 cells (each with three response categories measured), after synchronization of response onsets to 100 ms post-stimulus. It should be noted that the method of response onset synchronization was performed per cell and not per response category for each cell. Thus in principle the different categories of response could occur at different latencies in the Average Cell assessment. The Best, Mid and Worst responses, however, occur at the same latency. As can be seen, the shape obtained is 'qualitatively similar to the population PSTH profiles, except for a faster rise time. The response parameters measured from these PSTH profiles are shown in Table 3Jib.
It is important to note that the latency of onset for each of the three levels of response that has been detected for the data combined from 22 cells does not occur in advance of 100 ms. With a conservative latency assessment a small (possibly indiscriminate) response might have started in advance of the detected response onset. This would become evident in the average.
Discrimination between different levels of response and hence different types of stimuli was analysed by ANOVA of firing rate in different time bins across the 22 cells. This statistical evaluation of the discrimination is illustrated in Figure 3.6b. It is apparent that cells on average discriminate at statistically significant levels within the first 5 ms of response onset.
Variance in firing rate can be correlated with the response strength (Vogels and Orb an 1991). A suitable transformation [(firing rate + 1)1/%,
102 Z 82 0
1
62-g 42
=
22
+ + + + if- 57 84 111 138 166 193 220LATENCY (ms)
Fig u re 3.7. Sc a tt e r plo t o f l a te n c y a g a in stDISCRIMINATION DURING THE FIRST 100 MS OF RESPONSE.