Chapter 1 Vision from a Biological Viewpoint  What is the function of vision?

 Vision has a perceptual function.  It is through vision that we learn about the structure of our environment.

 Vision also allows us to perform skilled actions.  We are able to catch a butterfly or walk through a crowded room.

A Review of the Eye

The retina transduces light energy striking the rods and cones into signals that can be understood by the brain.

The 2 largest pathways from the eye to the brain in mammals are the retinotectal and the retinogeniculate. LGN V1 superior colliculus eye motor command eye

The Retinotectal Pathway  most important in birds, reptiles, and amphibians  terminates in the optic tectum or superior colliculus (SC) of the midbrain  the SC has pathways that lead to other areas of the brain such as the brainstem and higher-level visual areas in the cerebral cortex  the SC is involved in saccadic eye movements

The Retinogeniculate Pathway  also termed geniculostriate system  terminates in the dorsal part of the lateral geniculate nucleus of the thalamus (LGNd)  neurons in LGN project to the the primary visual cortex (V1)  the main pathway from the retina to the brain for vision in primates

How did the study of visuomotor systems begin?  Research traditionally focused on the input side of visual processing and virtually ignored its behavioral output.  Gibson (1977) recognized the importance of vision in the control of action.  According to Gibson, most sensory systems have exteroceptive (information about the outside events) and proprioceptive (information about one’s own actions) functions.

 The water beetle larva uses light to find the surface of the water in order to obtain air.  If placed in an aquarium that is illuminated from below, the larva will swim to the bottom of the aquarium and eventually suffocate.

 This link between light and the action of swimming to the light is a simple visuomotor mechanism. The input is linked to the output.  But, the larva will not swim to the light if it doesn’t need the air.

Does a similar input to output mechanism exist in higher organisms?  The first clear indication of the extent to which different visually guided behaviors are mediated by independent visuomotor pathways came from a ‘rewiring’ experiment carried out by Ingle in 1973.

 The optic tectum on one side of the brain was removed.  The frog appeared to be blind when stimuli were presented to the visual field contralateral to the missing optic tectum.  The frog’s optic nerve eventually regrew to the optic tectum on the opposite side of the brain! Prey-catching System

 When prey was presented to the visual field opposite the missing optic tectum, the frog snapped at a mirror image point on the other side.  When prey was presented to the eye opposite an intact optic tectum, the frog turned and snapped at it.

 When a barrier was placed in front of the frog, the frog was still able to maneuver around this barrier when startled from behind.  This result suggests that more than one retinal pathway exits. One pathway is responsible for the prey-catching system (retina to optic tectum) and the other for barrier avoidance (retina to pretectum).  Even though the prey-catching system was wired up the wrong way, the frog’s entire visual system was not reversed.

Subdivison of Tectal System  Turning towards a prey-like stimulus  Class II retinal ganglion cells to superficial laminae in optic tectum crossing over to nuclei in pons, medulla, spinal cord  Class II cells respond to small moving spots in visual field  Visual escape module  Class IV (some III) retinal ganglion cells to deeper tectal laminae uncrossed to nuclei in rostral medulla

Mammalian Vision  Modular organization is not limited to amphibians.  Traditionally, instead of examining the relationship between motor outputs and visual inputs, investigators working with mammals have typically looked at performance of subjects on visual discrimination tasks.  In these types of studies, it is assumed that the form of the motor response is not important, only the choice that the animal makes.

 Investigators have recorded eye movements while at the same time manipulating the characteristics of the controlling stimuli.  In this way, the activity of the single cells of the SC and other visuomotor structures have been shown to be related to the characteristics of the visual stimulus and the characteristics of the motor output.  There are some striking exceptions to this research tradition.

Visuomotor Modules in Mammals  The few studies that have looked at the visuomotor control systems in mammals have found that they are similar to those in amphibia.  Retinal projections to the SC appear to play a role in mediating brisk movements toward a target presented in the periphery.  This orienting behavior has been called the “visual grasp reflex”. It is a rapid shift in gaze that brings a stimulus originally located in the visual periphery into central vision.

 Lesions of the SC disrupt the animal’s ability to orient to visual targets.  This reflex is seen in rats even though it’s visual sensitivity varies only slightly across the retina. The movement may serve to position the stimulus so that it can be grasped with the mouth or forelimbs.  Stimulation of the SC elicits normal orienting movements.

 Stimulation will elicit movements resembling escape reactions.  Lesions of the SC reduce escape responses to threatening visual stimuli.  A 2nd projection system, from the retina to SC to targets in the ipsilateral brainstem, mediate visually elicited escape reactions similar to those in the frog.

 Finally, there is some evidence that a visual projection from the retina to the pretectum mediates barrier avoidance, much as in the frog.  Is this type of functional modularity similar in higher order mammals, such as primates?

 The world of a primate is much more complex and unpredictable than that of a frog. More flexible information processing is required.  Some visual areas are connected to systems associated with memory and planning and then actions are selected and performed.  In mammals, not all visual processing is linked directly to specific kinds of motor output.

2 Visual Systems Hypotheses  In the ‘60s, different functional dichotomies of the visual system were proposed, most of them contrasting the functions of the older pathway from the retina to the SC with the more recently evolved geniculostriate system.  Trevarthen (1968) suggested that the midbrain system mediates ‘ambiant’ vision and the geniculostriate system mediates ‘focal’ vision.  Ambient vision guided whole-body movements while focal vision guided fine motor acts.

 This distinction between object identification and spatial localization was incomplete.  He believed that prey-catching and barrier avoidance depended on the same localization mechanism in the SC.  Schneider (1969) argued that the retinal projection to the SC enables organisms to localize a stimulus in visual space, while the geniculostriate system allows them to identify that stimulus.

 Ungerleider & Mishkin (1982) concluded that appreciation of an object’s qualities depends on processing information in the inferior temporal cortex and appreciation of its spatial location depends on processing in the posterior parietal cortex. dorsal ventral

 Both of the ‘what’ and the ‘where’ pathways are contained within a perceptual framework rather than separating them into perception and motor orientation.  The evidence for this position was derived from behavioral experiments in which the visual discrimination abilities of monkeys with lesions in these 2 areas were tested and compared.

Parietal lesion disrupts “where” pathwayTemporal lesion disrupts “what” pathway  Monkeys with lesions in the inferior temporal cortex showed impairments in visual pattern recognition.  In contrast, monkeys with lesions in the posterior parietal cortex showed greater impairment in their ability to use a spatial ‘landmark’ as a discriminative cue in a choice task.

 Milner & Goodale suggest that the dorsal and ventral streams of vision do more than simply control the what and the where for perception.  They propose that the dorsal and ventral streams correspond to the functions of perceptual representation and visuomotor control. dorsal ventral