The dorsal stream’s principal role is to provide real-time “bottom-up” visual guidance of our movements online. In contrast, the ventral steam, in conjunction with top-down information from visual and semantic memory provides perceptual representations that can serve recognition, visual thought, planning and memory.
Schenk and McIntosh (2010) identified four core characteristics of the above two processing streams:
1 The ventral stream underlies vision-for-perception whereas the dorsal stream underlines vision-for-action.
2 Coding in the ventral stream is allocentric (object-centred; independent of the observer’s perspective), whereas dorsal coding is egocentric (body-centred; dependent on the observer’s perspective).
3 Representations in the ventral stream are sustained over time whereas those in the dorsal stream are short-lasting.
4 Processing in the ventral stream typically (but by no means always) leads to conscious awareness, whereas processing in the dorsal stream does not.
KEY TERMS
Ventral stream
The part of the visual processing system involved in object perception and recognition and the formation of perceptual representations.
Dorsal stream
The part of the visual processing system most involved in visually guided action.
Allocentric coding
Visual coding that is independent of the observer’s perspective.
Egocentric coding
Visual coding that is dependent on the observer’s perspective.
Finally, there are two other differences assumed theoretically between the two visual systems. First, processing in the dorsal stream is faster than in the ventral stream. Second, processing in the ventral stream depends more on input from the fovea (the central part of the retina used for detecting detail) than does dorsal processing.
Figure 2.9
Lesion overlap (purple = > 40% overlap; orange = > 60% overlap) in patients with optic ataxia. SPL = superior parietal lobule; IPL = inferior parietal lobule; SOG = superior occipital gyrus; Pc = precuneus.
From Vesia and Crawford (2012). Reprinted with permission of Springer.
Figure 2.10
(a) damage to DF’s lateral occipital complex within the ventral stream is shown in pale blue; (b) location of the lateral occipital complex in healthy individuals.
From James et al. (2003). Reprinted with permission of Oxford University Press.
Third, Pisella et al. (2009) argued that the notion that patients with optic ataxia have intact visual perception but impaired visually guided actions is over-simplified. When patients had access to visual feedback from their own hand, they only had severe problems with visually guided action in peripheral vision. There was much less evidence for impaired visually guided action in central vision, which is consistent with evidence indicating that many optic ataxics can drive effectively.
What about patients with damage to the ventral stream only? Of relevance here are some patients with visual form agnosia, a condition involving severe problems with object recognition even though visual information reaches the visual cortex (see Chapter 3). Probably the most-studied visual form agnosic is DF. James et al. (2003) found her brain damage was in the ventral pathway or stream (see Figure 2.10). DF showed no greater activation in the ventral stream when presented with drawings of objects than with scrambled line drawings. However, she showed high levels of activation in the dorsal stream when grasping for objects.
KEY TERM
Visual form agnosia
A condition in which there are severe problems in shape perception (what an object is) but reasonable ability to produce accurate visually guided actions.
In spite of having reasonable visual acuity, DF could not identify any drawings of common objects. However, DF “could accurately reach out and grasp a pencil orientated at different angles” (Milner et al., 1991, p. 424).
In another study (Goodale & Milner, 1992), DF held a card in her hand and looked at a circular block into which a slot had been cut. She could not orient the card so it would fit into the slot, suggesting she had very poor perceptual skills. However, DF performed well when moving her hand forward and inserting the card into the slot.
Dijkerman et al. (1998) assessed DF’s performance on various tasks when presented with several differently coloured objects. There were two main findings. First, DF could not distinguish accurately between the coloured objects, suggesting problems with object recognition due to damage to the ventral stream.
Second, DF reached out and touched the objects as accurately as healthy individuals using information about their positions relative to her own body.
This suggests her ability to use visual information to guide action using the dorsal stream was largely intact.
Goodale et al. (1994) gave DF two tasks. One involved distinguishing between two shapes with irregular contours and the other involved grasping these shapes firmly between thumb and index finger. DF performed very poorly on the former task that involved visual perception. However, Goodale et al.
concluded that DF “had no difficulty in placing her fingers on appropriate opposition points during grasping” (p. 604).
Himmelbach et al. (2012) argued that this conclusion is unwarranted. They reanalysed DF’s performance based on the data in Goodale et al. (1994) and compared it against that of 20 healthy controls (see Figure 2.11). DF’s performance on the grasping task was substantially inferior to that of the controls.
Similar findings were obtained when DF’s performance on other grasping and reaching tasks was compared against that of controls. Thus, DF has greater difficulties in visually guided action than was previously thought.
In sum, there are fascinating (and theoretically important) differences in visual perception and visually guided action between patients with visual form agnosia and those with optic ataxia. However, the picture is not neat and tidy. Both types of patients have problems with visual perception and with visually guided action. That complicates the task of making coherent sense of the findings.
Figure 2.11
Percentage of errors made when discriminating and grasping irregular shapes by DF (open circles) and by healthy controls (filled diamonds).
From Himmelbach et al. (2012). Reprinted with permission of Elsevier.
Interactive exercise:
Müller-Lyer