82 the angular and PL sensitivity of two ceils in one oamatiiium

of the loouat eye. successive penetrations with a single

electrode are usually achieved by gently tapping the electrode forward» which could easily alter the axis and rece^tive of uH oîuaaatilium slightly between >enetrations» so making the

aocoad recording ambiguous; this a proach was therefore not used k^lmultuneous responses from pairs ox cells proved exceedingly

difficult to record» but when obtained provided substantially aore iniormation than successive single records could have done.

i.ecordings were made with two aicroelectroles pro aligned just above the retina under the dissecting microscope» with

a

special jig (G. weiss» Jena)» so that the tips were about 5 pm apart, as the electrodes were driven together down through the eye surface» surface tension forces sometimes slightly altered the tip separation» so it was later found useful to cement the tips in position with ^oastaaa 91v Adhesive. .hen two intracellv recordings hixve been found by trial and error» the problem arise of whether to interpret these as being in one cell» in two

ceils in neighbouring oomatidia» or in two cells in the same ommatidium. It was not possible to directly nark even single cells reliably with the microaloctrode dye-marking methods

tried (nerkut and r.alker 1962» délirons and viulff 1965» Thomas and «ilson 1966) presumably because the electrodes have very fine tips. î^cvertUoless» two of the above alternatives can

33

to thô pair of ..Qcusta cells which gave the most complete information#

In Fig# 16 the angular respoaaea of two recording sites

are plotted in the vertical and horizontal plane through the receptive field centre» and both have the came "axis” or

position of peak response to within the nearest half-degree or so wmmatiiia in t M s central region of the eye have their geometries

aXGB separated on average by ^.5^ in the horizontal and 1^ in the vertical plane (Burtt and tatton 1)59» Tunstall and Horridge 1907) so the resolution of the technique enables us to sa/ with certainty that the two recordings did not come frcm separate o^matidia# The moot convincing evidence that this is so is that the irregularities in the upper curves in Fig# 16a» b are faithfully reflected in the lower curves: obviously both recordin sites are sharing the same optical apparatus» since the ccmaonly- occurring irregularities in locust retiaula receptive fields are quite different from cell to cell as an electrode advances throug a*i eye# This leaves two alternatives» but wo can rule out the possibility of both electrodes being in the same cell from Fig# IGc Cund also Fit^# I?). *wuen the polarized light response is plotted» the pianos of maximum soneitivity ere quite clearly different» about 6v^ apart# The records are therefore frjm separate cells in one ommatidium» and it appears that the pair have quite independent responses oecause of the hi^qh "sensitivity ratio" of each to different planes of polarization (defined in

X 3^® 180 I I 10'

Fi/r« 16 H#mpon#ee from two meparate calls in the #mm# locuat oinmatidlum* Tha #uce###iv# slow potential responses to short flashes appear as upward spikes because of the slow oscillosoepi sweep speed. In (a) the angular responses of the eglls are plotted by moving the small source horlsontally In 1 steps

between flashes, through the centre of the receptive field| (bj shows the angular response in the vertical plans through the visual field centre (dashed line). The receptive fields of thi

two cells are virtually Identical, even down to the small

irregularities. (c) the polarlsed-llcht responses are quite different, peaking about 60® apart. (d) Log intenslty-amplitiu curve for the two cells. Calibration pulses lOmV. Xnterflasl interval is 5 sec for (b), sec for (a), (c), (d), and flash length about )0 msec.

84,

& lethod#, 02)• fbe ratio was around 4#)%1 for the upper

retinuia and ^xl for the lower, by reference to the response- intensity curves (Fig# 161)#

The extent of independence of this pair of cells was also assessed by testing the degree of their electrical inter­ connection# The Wheatstone bridge circuit is balanced in the daj for depolarising current pulsed into the top cell (Fig# IVa), and records a ^axiaal fall in uiembrane resistance of about Iv BJ\

during the light flash# In the dark, a JO potential shift of about 1 mV is recorded in the lower cell (arrow) as a result of current spread from the top cell, but this potential altkost

disappears during illumination, recovering as aemurane resistance of each cell recovers fallowing the flush# The reciprocal

process occurs when current is pulsed into the bottom cell (Fig# 17b)# dy contrast, no JO derivative potentials ore seen if the two cells are in adjacent os&matidia (Fig# l?d). Judged on the

basis of distinct receptive fields#

The "coupling ratio” of cell pair is maximally unity if defined as the ratio of the steady potential shift U 2^

produced in the second cell relative to that in the first

during the current pulse, and can be determined onlj if we have some way of estimating V^‘# This is not straightforward with sinc^le microelectrodes since the vol;^age appearing at the pro­ amplifier is largely due to current passing down the microelectr% resistance, and this varies during current passage, besides bein.

a

e

Fig» 17# (a-c)# Tha sama pair of call# a# Fig# l6# In (a)#

depolarizing current puleaa are paasad into the top cell# and

produca small derivativa potential shifts in the dark in the

other call (arrow); the reverse process is shown in (b). Fro:

the size of these pulses and the maximum extant of bridge

imbalance during the light flash, the coupling between calls cai