j *@ - Sociedade Brasileira de Psicologia

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of the normalized intensity function per 1 1og intensity step) of about 85%#og unit,
whereas photoreceptors #ve values of about 35%/1og unit. Increasing light intensity
above saturation adlu ly decreases the response in most cells, confirming Menzel's
observation.
Attempts to measure the sm ctral sensitivity function S ( ) by means of the
constant response method (Menzel, Ventura, Sola , Hertel, Greggers, 1986) were
successful in only a few cells. Inly cells wluch respond with a sustained plateau can be
clamped to a constant response and due to the high noise level in MCs a relatively large
plateau potential ks also necessary. Thb S ( ) functions obtained were similar to those
of green/ensitive receptor cells but with peaks which varied between 505 and 535 nm
and secondary peaks in the UV. However, stability was not sufficient to obtain clear
cut results. In most other cels three flashes (544, 444, and 344 nm) were used to
evaluate the most sensitive spectral region; the majority were green sensitive, but a
few blue- and Uv-sensitive cels were registed for peak response. For plateau response
there was a greater incidence of Uv-sensitive cels.
Many wel defined markings were obtained the majority of which showed
morphology similar to Ribi's 12 type: many bilateral branches in the A layer of the
lamina, little or no branching in the B layer and none in the C layer. Forked and
branching terminals in. the distal medulla. Two or perhaps 3 cels were similar to Ribi's
L1 type TbranchinG in a1l three layers of the lamina and less developed terminals in the
snme region of the medulla. One of these had extended spines in the A layer of the
lnmina not shown by Ribi; this wàs the depolarizing cell mentioned above. A11 three
spiking cels were of the 1.2 type.
DB CUSSION
The maximum amplitude of MC responses is considçrably smaler (15 - 20
mV) than those of the photoreceptors (ca. 50 mV). Howcyer, the peak responses to
smal increments or decrements of intensity are actllnlly m uch larger. How then is the
entire dynam ic range covered by the photoreceptors processed in the MC stage? In fly
and dragonfly lazge monopohr cels (LMC's) Ixqugltlin and Hardie (1978) have shown
that rapid adaptation maintains the résponses to changes in intensity in the central
linear region of the V/logl curve and leeps contrast eficiency at high levels. Thtks the
smal dynamic range of the steep V/lèg I function does not become a lirniting factor,
since in most normal situations changes in the intensity of illtmination occur in sm al
increm ents above and velow the mean to which the MC's are constantly adapted .
With few exceptions the responses of the MCs reportcd here did not adapt
quickly. Could the bee be different in this respect? If this be thc case the MCs would
saturate within thc dynamic range of the photoreceptors. Most likely this apparent
contradiction is due to the tlse qf a point source here whereas La . ughlin & Hardie used