Consciousness

• seCtIon tHRee: BoDY AnD WoRLD
  a piece of card she could quickly and accurately post it
  through the slot  – a task requiring accurate alignment.
  How can this be? How can she be unaware of the angle
  of the slot and yet able to post the card into it? The
  answer, according to Milner and Goodale, is that she has
  lost much of the ventral stream that leads to visual per-
  ception but retains much of the dorsal stream needed
  for accurate visuomotor control  – losses which have
  been confirmed by multiple scans (Whitwell, Milner, and
  Goodale, 2014).
  According to Milner and Goodale, these experiments
  show that sometimes ‘what we think we “see” is not what
  guides our actions’ (1995, p. 177) and that ‘the visual
  signals that give us our experience of objects and events in the world are not
  the same ones that control our actions’ (Goodale, 2014). These findings were
  subsequently challenged (Franz et al., 2000), reanalysed to meet the challenge
  (Danckert et al., 2002), and much debated, with an alternative proposal that
  visual illusions affect the planning of actions but not their online control (Glover,
  2002; Goodale, 2007).
  These dissociations are not limited only to brain-damaged patients. The
  same separation between perception and motor control was reported in a
  study using visually normal participants tricked by a visual illusion (Aglioti,
  Goodale, and DeSouza, 1995). Thin discs were made to look different sizes by
  surrounding them with rings of larger or smaller circles. This is the Ebbing-
  haus or Titchener illusion, which has been shown to trick even fish (Sovrano,
  Albertazzi, and Salva, 2014) and chicks (Salvo et al., 2013) as well as humans,
  suggesting it taps into a feature of cognition that has a broad evolutionary
  basis.
  Participants had to pick up the left-hand disc if the two discs appeared equal
  in size and the right-hand disc if they appeared different, for many different
  sizes and apparent sizes of discs. The aperture of their finger–thumb grip
  was measured as they did so, allowing motor performance and a perceptual
  decision to be measured in the same task. Participants saw the usual size illu-
  sion (as shown by their choice of disc), but their grip fitted the actual disc.
  Apparently the visuomotor system was not fooled, although the perceptual
  system was. The same illusion has since been demonstrated in the tactile
  modality, with blindfolded participants exploring foam cut-outs of the circles
  using touch (Ziat et al., 2014). And other work with healthy participants has
  shown that modifying the subjective experience of masked stimuli can be
  done without changing on the motor effects of those stimuli (Vorberg et al.,
  2003).
  These studies underline the important difference between processing for per-
  ception and processing for motor control. The distinction makes sense in evo-
  lutionary terms because the constraints on the two systems are different. Fast
  and accurate responses to changing visual stimuli are essential for catching
  prey, avoiding dangers, and even basic abilities like standing upright. By con-
  trast, object identification can wait. Rich detail rather than speed may be more
  important when planning future actions and making strategic decisions, and this

‘what we think we “see”

is not what guides our

actions’

(Milner and Goodale, 1995,
p. 177)

LIP

7a MT

V2

V1

V4

PIT

AIT

Dorsal stream (parietal)

Ventral stream (temporal)

FIGURE 8.9 • The ventral and dorsal visual
streams. Ungerleider and Mishkin
called them the ‘what’ and
‘where’ streams. Milner and
Goodale suggest that they carry
out vision for perception and vision
for action (or visuomotor control)
respectively (Milner and Goodale,
1995, p. 22).