The fMRI response of the LGN and V1 to cardinal red-green, blue-yellow, and achromatic visual stimuli

We compared the responsiveness of the LGN and the early retinotopic cortical areas to stimulation of the two cone-opponent systems (red - green and blue - yellow) and the achromatic system. This was done at two contrast levels to control for any effect of contrast. MR images were acquired on seven subjects with a 4T Bruker MedSpec scanner. The early visual cortical areas were localised by phase encoded retinotopic mapping with a volumetric analysis (Dumoulin et al, 2003 NeuroImage 18 576 - 587). We initially located the LGN in four subjects by using flickering stimuli in a separate scanning session, but subsequently identified it using the experimental stimuli. Experimental stimuli were sine-wave counterphasing rings (2 Hz, 0.5 cycle deg-1), cardinal for the selective activation of the L/M cone-opponent (RG), S cone-opponent (BY), and achromatic (Ach) systems. A region of interest analysis was performed. When presented at equivalent absolute contrasts (cone contrast = 5% - 6%), the BOLD response of the LGN is strongest to isoluminant red - green stimuli and weakest to blue - yellow stimuli, with the achromatic response falling in between. Area V1, on the other hand, responds best to both chromatic stimuli, with the achromatic response falling below. The key change from the LGN to V1 is a dramatic boost in the relative blue - yellow response, which occurred at both contrast levels used. This greatly enhanced cortical response to blue - yellow relative to the red - green and achromatic responses may be due to an increase in cell number and/or cell response between the LGN and V1. We speculate that the effect might reflect the operation of contrast constancy across colour mechanisms at the cortical level.

Jules Cotard (1840-1889) - His life and the unique syndrome which bears his name

Dr. Jules Cotard (1840-1889) was a Parisian neurologist who first described the delire des negations. Cotard's syndrome or Cotard's delusion comprises any one of a series of delusions ranging from the fixed and unshakable belief that one has lost organs, blood, or body parts to believing that one has lost one's soul or is dead. In its most profound form, the delusion takes the form of a professed belief that one does not exist. Encountered primarily in psychoses such as schizophrenia and bipolar disorder, Cotard's syndrome has also been described in organic lesions of the nondominant temporoparietal cortex as well as in migraine. Cotard's delusion is the only self-certifiable syndrome of delusional psychosis. Jules Cotard, a Parisian neurologist and psychiatrist and former military surgeon, was one of the first to induce cerebral atrophy by the experimental embolization of cerebral arteries in animals and a pioneer in studies of the clinicopathologic correlates of cerebral atrophy secondary to perinatal and postnatal pathologic changes. He was the first to record that unilateral cerebral atrophy in infancy does not necessarily lead to aphasia and was also the pioneer of studies of altered conscious states in diabetic hyperglycemia.