Simultaneous recording from two tectal locations was achieved in 5 animals with a total of 24 MURFs. This included 6 pairs from overlapping receptive fields and 6 pairs from non-overlapping receptive fields (figure 4.4). In each of these groups 2 of the pairs were recorded from opposite tectal lobes and the other 4 were recorded from the same tectal lobe. Before the correlated firing patterns of the MURFs were examined the receptive fields were marked out on a perimeter card using a 3cm diameter black disc to identify the receptive field size of the MURFs (26.6° ± 9.0°sem, n=19) and their position in visual space. The result of visual stimulation with a black disc during normal light conditions is shown for a pair of overlapping and non-overlapping receptive fields recorded from two tectal locations in shown in figure 4.4. No qualitative difference was observed between those pairs recorded from opposite or the same tectal lobes. It should be noted that there is no cross-talk between the two recording sites and the signal to noise ratio is sufficient in both cases to identify the time of individual action potentials present in the MURFs. Cfoss-correlations (CCs) calculated from the firing patterns of overlapping receptive field responses to visual stimuli reveals a broad peak at t = 0 seconds (figure 4.5) irrespective of whether they were recorded from the same or opposite tectal lobes. CCs from non-overlapping receptive fields were flat with no cental peak indicating, not surprisingly, no correlation between the two firing patterns.
M U R F ' s 0. 5mV 10 0 m s e c M U R F ’ s O.SmV 2 5 0 m s e c
F igure 4.4. Exam ples o f sim ultaneous extracellular recordings m ade fro m th e sam e a n d opposite tectal lobes during stim ulation of non-overlapping a n d overlapping visual receptive fields.
The top two traces are simultaneously recorded from two separate locations on the same tectal lobe (as depicted diagramatically by the inset). Stimulation o f receptive field A, with a black disk, results in action potentials o f varying amplitudes in trace A with no response recorded in B. The opposite effect was observed when receptive field B was stimulated (not shown). The receptive fields o f these multi-units were plotted on perimetric mapping co-ordinates (see chapter 3) to determine their size and location in visual space. Note that no artifactual cross-talk is apparent when the recording locations are in the same tectal lobe.
The bottom two traces were recorded simultaneously from opposite tectal lobes during stimulation o f overlapping receptive fields with a black disk. In this case neuronal activity is recorded at both tectal locations in response to visual stimulation o f the overlapping visual fields. The time base is longer (250msec) to allow the documentation o f two consecutive visual stimulations. As the recordings are made from opposite tectal locations the pathways giving rise to these events are entirely separate. However, the time at which the activity occurs at both tectal locations seems to be correlated.
N o n - o v e r l a p p i n g f i e l d s
O v e r l a p p i n g f i e l d s
1 0 0 n
5 0 -
Figure 4.5. C ross-correlatioiis calculated from non-overlapping and overlapping receptive fields d u rin g visual stim ulation.
E vent data w a s recorded sim u lta n e o u sly at two tectal lo cations (channel A & B) in res p o n se to a bla c k disk m oving th r o u g h the re cep tiv e field. T h is timing data w a s used to c o n s t ru c t a c r o s s c o r r e la t io n betw een the tw o separate sets o f ev ents . T h e a lg o r ith m a d o p te d to c o n s tru c t the c r o s s c o r r e la t io n can be s u m m a ris e d ; for ea ch ev en t in A calculate the tim e ( o v e r a p e r i o d o f ± 0 . 5 seconds) at w h ic h events o c c u r in B. T h is w a s calc ulated for all events in A w h ic h o c c u r r e d o v e r a 20 second p e ri o d d iv id in g the da ta into 500/rseco n d time bins. T h e c ro s s -c o rr e la tio n s d e s c r ib e the te m poral r e la ti o n s h ip b etw een e v e n ts in A relative to those even ts in B..
F o r each plot the ab scissa s h o w s the time in seco nds ( ± 0 . 5 s e c ) at w h ic h e v e n ts w e re recorded. T h e n u m b e r of ev en ts recorded in each o f these 500/rsecond time b in s is sh o w n on the o rd in a te scale. W h e n the tw o M U R F s are n o n -o v e r la p p in g in visual space s tim u la tio n o f o n e re cep tiv e field c au ses no a c ti \’ity at the oth er tectal location. T h e cross-correlation is Hat w ith no central peak s u g g e s tin g that n eu ro n al activity at the tw o tectal locations is not correlated. W h e n the tw o M U R F s are o v e rla p p in g activ ity at the two tectal locations is correla ted. Stimulation o f the o \ e r l a p p i n g p o rtio n o f visual space cau ses acti \ ity at both tectal locatio ns. T h e re fo r e , a central peak is p r o d u c e d in the cro s s-c o rr e la tio n a r o u n d 0 seco nds.
4.2.3. The acute effects of IHz stroboscopic illumination on correlated firing