A teleost eye with a grouped retina – Spatio-temporal properties of the visual system of Gnathonemus petersii
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1
University of Bonn, Zoology, Germany
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2
Bielefeld, Germany
The mormyrid fish Gnathonemus petersii is renowned for its ability to perceive self-generated weakly electric fields for orientation, foraging and communication. This is seen as an adaptation to a nocturnal lifestyle and turbid habitats. However, Gnathonemus also evolved a highly specialized visual system containing bundled photoreceptors, a tapetum lucidum, and retinomotoric activity. The aim of this study was to characterize early-stage neural processing of visual information and to determine the retino-tectal projections to reveal the adaptive advantage of this peculiar retinal arrangement.
We measured the temporal contrast sensitivity function to determine contrast dependent flicker fusion frequencies (FFF) in the major retinorecipient area, the optic tectum. We found FFF up to 50Hz, with best responses to frequencies between 15 and 20Hz. The FFF obtained in goldfish was lower, ranging up to 40Hz, with best responses at 5Hz.
With extracellular recordings, we characterized properties of single tectal units, most likely terminals of retinal afferents. Their receptive field (RF) sizes were measured and compared to the predicted visual resolution based on bundle diameter and the anatomical organization of retinal ganglion cells. The spatial resolution of tectal units was low: RF sizes ranged from 6° up to 50°. Different response properties of units were found: FAST-, ON-OFF-, ON- and OFF-responses.
Responses to rotating grating stimuli of different velocities and sizes were recorded. Tectal units responded to gratings of low spatial frequencies up to 0.33cycle/deg and velocities up to 30Hz.
Further, the central representation of the visual field of Gnathonemus was determined. The spatial organization within the optic tectum was found to be linear and did not show any specialized regions corresponding to an over- or under¬representation of parts of the visual field.
Our results suggest that the bundled photoreceptor arrangement combined with the light gathering tapetum enables G. petersii to use the high temporal resolution of the cone pathway under dim light conditions. The tapetum focuses the light, resulting in an increased intensity at the cone outer segments. The low spatial resolution of the visual system - due to the size of a receptor bundle - can be regarded as a low-pass filter cancelling out interferences from small objects.
Anatomical findings in connection with our physiological results support the hypothesis of a specialized retinal structure in Gnathonemus petersii: This new type of retinal organization enables the reliable and vital use of a fast visual system in dim and turbid habitats. The advantage of this specialization might be visual far-field predator avoidance outranging the electric sense of the animal.
Keywords:
Carassius auratus,
contrast sensitivity function,
critical flicker fusion frequency,
sensory ecology,
temporal resolution,
visual evoked potentials
Conference:
Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012.
Presentation Type:
Poster Presentation (see alternatives below as well)
Topic:
Sensory: Vision
Citation:
Pusch
R,
Von Der Emde
G and
Engelmann
J
(2012). A teleost eye with a grouped retina – Spatio-temporal properties of the visual system of Gnathonemus petersii
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Conference Abstract:
Tenth International Congress of Neuroethology.
doi: 10.3389/conf.fnbeh.2012.27.00240
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Received:
30 Apr 2012;
Published Online:
07 Jul 2012.
*
Correspondence:
Dr. Gerhard Von Der Emde, University of Bonn, Zoology, Bonn, 53115, Germany, vonderemde@uni-bonn.de