Color vision and Frithiof Holmgren's discordant retinal microstimulation findings.

Frithiof Holmgren was a nineteenth-century Swedish professor of physiology who, after a sabbatical year at Helmholtz's laboratory in Heidelberg, devoted himself to studies of color vision and color blindness. Following Blix's successful demonstration of cutaneous receptor specificity by means of point stimulation, he decided to apply an analogous technique for psychophysical examination of the human retina. His findings appeared to support the Young-Helmholtz hypothesis of three primary colors and invalidated Hering's alternative four color opponent hypothesis. However, contemporaries (Hering, Isaachsen) could not reproduce Holmgren's results, and his findings fell into disrepute. They have now been confirmed, but their previous theoretical significance was not supported.

Cortical processing of lateral skin stretch stimulation in humans.

Direction discrimination of a moving tactile stimulus requires intact dorsal columns and provides a sensitive clinical test of somatosensory dysfunction. Cortical mechanisms are poorly understood. We have applied tangential skin pulls to the right lower leg during functional magnetic resonance imaging. Healthy subjects judged the direction of the skin pulls (task experiment, n = 7) or received skin pulls passively (no task experiment, n = 8). Second somatosensory cortex (S2) was activated in the task as well as no task experiment, and there was no significant difference in cortical activation between the two experiments. Within S2 nearly all subjects had prominent activations in the caudal and superficial part, i.e., in the opercular parietal (OP) area 1. S1 was activated in only one of the subjects. Thus, S2 and especially OP 1 seems to be important for processing of lateral skin stretch stimulation. The finding suggests that a lesion of this area might cause a disturbance in tactile direction discrimination which should be relevant for clinical testing.

Tactile directional sensitivity and postural control.

People are good at telling the direction of a moving tactile stimulus and this capacity provides a sensitive clinical test of somatosensory disturbances. Tactile directional sensitivity depends on two different kinds of somatosensory information, i.e. spatiotemporal information and information about friction-induced changes in skin stretch. The objective of this study was to compare the relative contribution to postural control of these two types of information for both glabrous and hairy skin. Postural sway amplitudes and sway paths were recorded, with or without access to tactile and/or visual stabilizing stimuli. Subjects were standing on two types of surface, either solid metal or 50 mm foam plastic. Two types of stimulus were used to generate sway-related tactile information. One was a thin air-stream that was used to assess the contribution by spatiotemporal information, and the second was a narrow steel rod that was glued to the skin to assess the contribution by skin-stretch information. The stimuli were applied to the hairy skin of the forearm and to the glabrous skin of the fingertip. In addition, we studied the ability to tell the direction of movement of an air-stream stimulus on glabrous and hairy skin. The air-stream caused significant sway reductions when applied to glabrous, but not hairy skin. The weak effect on hairy skin reflected the perceptually poor directional sensitivity for the air-stream stimulus in this cutaneous area. In contrast, the glued rod reduced sway when applied to both glabrous and hairy skin reflecting the tactile afferents' high sensitivity to skin stretch in these areas. Both types of tactile stimulus reduced sway amplitudes more than sway paths for both hairy and glabrous skin. The visual cue, on the other hand, tended to reduce sway paths more than amplitudes. The two types of tactile receptive surface seem to influence postural control in the same manner, despite anatomical and physiological differences. The results invite speculation that patients with poor directional sensitivity might have reduced postural stability compared with healthy individuals.

White matter disease in magnetic resonance imaging predicts cerebral complications after coronary artery bypass grafting.

BACKGROUND: The aim of the present study was to assess neurologic and neuropsychologic complications in 104 patients randomized to coronary artery bypass grafting or spinal cord stimulation. An additional objective of the study was to assess whether preoperative white matter disease might predict cerebral complications, as previous studies have shown that there is a relationship between white matter disease and neuropsychologic decline after coronary artery bypass grafting. METHODS: The patients were subjected to neurologic examination before and six months after intervention. The patients underwent a cerebral magnetic resonance imaging before intervention and the presence of white matter disease was related to development of cerebral complications. RESULTS: More patients in the bypass group than in the spinal cord stimulation group developed focal cerebral ischemia (p < 0.05) and astheno-emotional disorder (p < 0.001). More patients with white matter disease undergoing bypass were affected by focal cerebral ischemia (p < 0.01) and astheno-emotional disorder (p < 0.001) after the intervention compared to patients with white matter disease undergoing spinal cord stimulation. In patients with no white matter disease there were no differences between the bypass group and spinal cord stimulation group with regard to cerebral complications. CONCLUSIONS: Patients undergoing bypass had more neurologic and neuropsychologic complications than patients undergoing spinal cord stimulation. Furthermore, patients with white matter disease were affected by cerebral complications in a higher extent after bypass than after spinal cord stimulation. Thus, preoperative assessment of white matter disease before undergoing coronary artery bypass grafting might predict the patient's risk of developing cerebral injury.