Neuronal nonlinearity explains greater visual spatial resolution for darks than lights.

Astronomers and physicists noticed centuries ago that visual spatial resolution is higher for dark than light stimuli, but the neuronal mechanisms for this perceptual asymmetry remain unknown. Here we demonstrate that the asymmetry is caused by a neuronal nonlinearity in the early visual pathway. We show that neurons driven by darks (OFF neurons) increase their responses roughly linearly with luminance decrements, independent of the background luminance. However, neurons driven by lights (ON neurons) saturate their responses with small increases in luminance and need bright backgrounds to approach the linearity of OFF neurons. We show that, as a consequence of this difference in linearity, receptive fields are larger in ON than OFF thalamic neurons, and cortical neurons are more strongly driven by darks than lights at low spatial frequencies. This ON/OFF asymmetry in linearity could be demonstrated in the visual cortex of cats, monkeys, and humans and in the cat visual thalamus. Furthermore, in the cat visual thalamus, we show that the neuronal nonlinearity is present at the ON receptive field center of ON-center neurons and ON receptive field surround of OFF-center neurons, suggesting an origin at the level of the photoreceptor. These results demonstrate a fundamental difference in visual processing between ON and OFF channels and reveal a competitive advantage for OFF neurons over ON neurons at low spatial frequencies, which could be important during cortical development when retinal images are blurred by immature optics in infant eyes.

Faster thalamocortical processing for dark than light visual targets.

ON and OFF visual pathways originate in the retina at the synapse between photoreceptor and bipolar cells. OFF bipolar cells are shorter in length and use receptors with faster kinetics than ON bipolar cells and, therefore, process information faster. Here, we demonstrate that this temporal advantage is maintained through thalamocortical processing, with OFF visual responses reaching cortex ~3-6 ms before ON visual responses. Faster OFF visual responses could be demonstrated in recordings from large populations of cat thalamic neurons representing the center of vision (both X and Y) and from subpopulations making connection with the same cortical orientation column. While the OFF temporal advantage diminished as visual responses reached their peak, the integral of the impulse response was greater in OFF than ON neurons. Given the stimulus preferences from OFF and ON channels, our results indicate that darks are processed faster than lights in the thalamocortical pathway.

NMDA receptors are involved in Ginkgo extract-induced facilitation on memory retention of passive avoidance learning in rats.

Herbal therapies are commonly used to enhance memory and learning. Ginkgo biloba has shown to be one of the most popular herbs that is used to treat amnesia and retard age related memory deficits. Although, there have been several reports on the memory enhancing effects of Ginkgo, involvement of glutamatergic system that plays pivotal role in learning and memory has not been precisely assessed so far. The current study intended to investigate the effect of Ginkgo intake on amnesia while NMDA (N-methyl D-aspartic acid) receptors blocked by the administration of MK-801. The study used passive avoidance (PA) task to investigate the effect of chronic administration of Ginkgo extract (40 and 90 mg/kg; oral) on the memory span in male Wistar rats, suffering from MK-801-induced forgetfulness (0.06 and 0.1 mg/kg; i.p.). The results indicate that Ginkgo was able to remove MK-801-induced forgetfulness, indicating that Ginkgo can affect memory retention but not effect on passive avoidance acquisition, using pathways other than glutamatergic system as well. The results might indicate that Ginkgo extract can be effective in removing forgetfulness caused by inhibiting NMDA receptors from performing their activities.