SUMMARY1. Pairs of goldfish retinal ganglion cells with overlapping receptive fields were recorded during stimulation with repeated light flashes. Cross-correlation histograms for 'maintained' discharge, 'on' responses, and 'off' responses were computed with a correction for the systematic responses to the stimuli; cross-covariances were derived from these. If stimulus-induced signals and noise combine linearly, then the cross-covariances are independent of differences in mean firing rate.2. Cross-covariances of pairs of cells with the same response polarity displayed a positive peak near zero lag; pairs with complementary responses showed a negative peak. 'On-off' cells could generally be classified as on-like or off-like, based on the plateau of firing during a prolonged flash and the relative magnitudes of the on and off peak responses; the cross-covariances of these cells were as one would predict if they were pure on-or off-centre neurones.3. The cross-covariances derived from the on period usually differed in magnitude from those derived in the dark (either maintained or off response). In general, cross-covariances for off responses were nearly identical to those for the maintained discharges ofthe same pair, although the mean rates at off were usually quite different from the maintained. The change in magnitude of the cross-covariances from on responses therefore appears to be a non-linear effect of light, and not of the changes in firing rate induced by the light.4. Other features of the cross-covariances were not affected by stimulation. The general shapes remained fairly constant, and the lags at which the peaks occurred were not consistently affected.5. We estimated the variance of the firing rate of each unit in three ways, and used two methods of portioning the variance implied by the cross-covariances; from these estimates, we obtained an upper bound for the proportion of the variance of firing of a cell which is due to the common noise that affects both members of a pair. We found that the common influence accounts for less than 20 % of the total variance.During stimulation, both the magnitude of the cross-covariance and the variance of the rates change; however, the percentage of total variance contributed by the common noise source is constant.

SUMMARY1. Pairs of retinal ganglion cells in the isolated goldfish retina were recorded simultaneously with a single electrode. Repeated flashes of light were delivered to evaluate the response latency of each of the units.2. The cross-correlation histogram for the maintained discharge ofeach pair of cells was examined, and its temporal relationships (lags) were compared with the differences in response latencies of the two units. There was a strong correlation between these measures; however, the differences between latencies were often at least twice as great as the lags.3. The differences between the times to the peaks of the responses of the two units were less reliably related to the lags of the pairs, although the correlation was positive and the differences in time-to-peak generally greater than the lags. The weaker relationship between the difference in time-to-peak and lag than between latency difference and lag is apparently a manifestation of a negative correlation between latency and rise time (from first response to peak). This indicates that cells with a longer latency compensate with a faster rise time.4. There was a negative correlation between the mean maintained rate of a neurone and its response latency. That is, cells with faster maintained discharge rates respond sooner than those with slower maintained rates.5. There was virtually no relationship between the lags or the differences in latency and the differences between the magnitudes of the responses to light. Thus, it is unlikely that differences in latency (or lags) could be attributed to unequal effectiveness of the stimuli for the two units.6. The relationship between differences in latency and lags did not depend on the response categorizations of the two units. Specifically, it did not matter whether the members of the pair were on centre, off centre or on-off centre; neither did it matter whether they were X-like or not-X-like neurones. 7. Consideration of these data leads to the conclusion that there must be 'marked' pathways of differential conduction velocity through the retina.

We hypothesized that the corticospinal system undergoes functional changes in long-term polio survivors. Central motor conduction times (CMCTs) to the four limbs were measured in 24 polio survivors using transcranial magnetic stimulation (TMS). Resting motor thresholds and CMCTs were normal. In 17 subjects whose legs were affected by polio and 13 healthy controls, single- and paired-pulse TMS was used to assess motor cortex excitability while recording from tibialis anterior (TA) muscles at rest and following maximal contraction until fatigue. In polio survivors the slope of the recruitment curve was normal, but maximal motor evoked potentials (MEPs) were larger than in controls. MEPs were depressed after fatiguing exercise. Three patients with central fatigue by twitch interpolation had a trend toward slower recovery. There was no association with symptoms of post-polio syndrome. These changes occurring after polio may allow the motor cortex to activate a greater proportion of the motor neurons innervating affected muscles.

BackgroundMotor fatigue is a common complaint in patients with chronic neurological disorders such as post-polio syndrome and multiple sclerosis. Fatigue occurs via central and peripheral mechanisms. Central fatigue is caused by decreased output from the motor cortex to spinal motor neurons. Transcranial magnetic stimulation (TMS) is a non-invasive technique used to study the intracortical networks that may produce fatigue. Motor evoked potential (MEP) amplitude is a measure of the motor cortex output and functional state of the pre-motor inhibitory and facilitatory circuits. During sub-fatiguing exercise, there is facilitation (MEP amplitude increases). Central mechanisms underlying this phenomenon are not known. A possibility is that intracortical networks mediate post-exercise MEP facilitation.ObjectivesOur aim was to examine intracortical inhibition (ICI) and facilitation (ICF) during sub-fatiguing exercise, using paired-pulse TMS.MethodsThirteen healthy adults ages 25-80 were recruited for this experiment. Subjects performed 6 sets of sub-fatiguing isometric exercise of the abductor digiti minimi muscle for 30 seconds at 50% maximal voluntary contraction. Muscle force was measured via a force transducer with continuous visual feedback. After each exercise set, the contralateral motor cortex was stimulated with 5 pulses of paired-pulse TMS at a rate of 0.2 Hz. MEPs were recorded by surface electromyography. TMS conditioning and test stimulus intensities were 75% and 120% of the resting motor thresholds respectively. Interstimulus intervals of 3 and 10 ms were used to measure ICI and ICF ratios.ResultsThe mean resting unconditioned MEP amplitude was 0.73 ± 0.69 mV. Resting ICI and ICF ratios were 0.4 ± 0.25 and 2.3 ± 1.4 respectively. After exercise, MEPs increased to 2.30 ± 1.08 mV (p < .0001). ICF ratios significantly decreased by 48% to 1.1 ± 0.4 (p = .003). ICI ratios were unchanged (0.4 ± 0.3 to 0.28 ± 0.1).ConclusionsWhile MEP amplitudes tripled during sub-fatiguing exercise, the ICF ratio decreased. This suggests that facilitation maximizes intracortical facilitatory networks such that there is a decreased reserve for further facilitation. ICI ratios were unchanged during exercise, suggesting that reduced activity in inhibitory networks is not affected in the same manner. Our findings suggest that healthy adults may draw upon a reserve of facilitatory intracortical networks during exercise to increase the output of the motor cortex. We hypothesize that patients with central fatigue may have a reduced ability to recruit this reserve. Investigation of other intracortical mechanisms such as long interval ICI and short interval ICF may supplement these findings.