19.4  Verbal Mapping

Though mapping of phosphenes by retinal and optic nerve stimulation was reported long after the first cortically evoked phosphenes were characterized, the methods used were not very different. Several papers by Humayun et al. mention mapping of retinally evoked phosphenes [18–20], but they do not provide detailed information about mapping conditions, such as gaze-control of the subject or tactile references. In acute experiments Humayun et al. constructed absolute maps by asking the subject to verbally inform the experimenter of the quadrant (1–4), or clock hour (e.g., “9 o’clock”) in which the phosphene was perceived [18]. As expected, the results confirmed that subjective phosphene location corresponded well with the electrically stimulated area on the retina. On the basis of these findings, they extended their observations to relative measures by providing simple patterns through multi-electrode probes on the retinal surface and asking the subject about their percepts in acute experiments [19].

Verbal mapping was also applied in a study using TMS to evoke phosphenes in sighted subjects [27]. Subjects were placed in a darkened room with eyes closed to facilitate phosphene percepts elicited by stimulation of the occipital lobes. Subjects reported if the phosphenes appeared in the upper or lower visual field, and whether the percepts were centrally or peripherally located. It appeared that peripheral phosphenes were encountered more often than central ones. Furthermore, phosphenes were more often observed in the lower-field than in the upper field. The first finding is unexpected, since the central visual field at the occipital pole (i.e. the foveal projection) is more accessible for TMS than the peripheral retinal projections located at more rostral aspects from the calcarine fissure. The authors speculated that TMS in this study may have activated peristriate cortical areas.

Fernandez et al. [14] proposed an alternative method of phosphene imaging by verbal communication for sighted people. This method incorporates several training phases by using a clock-face division of visual space. Each of the 12 sectors is labeled accordingly and divided into annuli to produce an inner, middle, and outer portion, representing displacement between the fovea and visual periphery. In the training phase subjects are provided with a computer screen and learn to specify the projection of a light spot over the clock-face frame. Initially, spots of light are presented onto a full outline of the frame and subjects are asked to indicate in which of the 36 sections the spot appears (hour and eccentricity). The second training phase is performed without sector labeling and subjects receive feedback about their performance. The last phase consists of phosphene localization without the frame and subjects again receive feedback on their performance. Each of the training phases are repeated until the subject achieves a required percentage correct performance level, before proceeding to the next phase. After training, testing consists of identification of the location of phosphenes in an imaginary frame. The authors predict that this method should be faster and should yield a higher spatial resolution and more discrete responses from the subjects than other absolute mapping methods. In addition, any effects of visuo-motor transformations required for drawing are eliminated by this method.

The authors mention that blind people could learn this method (though training with a visible frame would be impossible) with the help of a dartboard divided into 12 sectors and three annuli, much like the method of Mladejovsky et al. [26] discussed above.

19.5  Mapping Studies Using Subject Drawings

After their first studies on phosphene mapping using acute retinal stimulation (Sect. 19.3), Humayun et al. chronically implanted a retinitis pigmentosa patient without functional vision with a 16-electrode retinal implant [20]. They mapped the phosphenes by letting the subject draw the percepts on a drawing board positioned on the subject’s lap. Similar to their preceding studies using verbal information discussed above, the constructed phosphene maps indicated that percepts correspond well with the retinal layout; i.e., electrodes temporally located evoked nasally perceived phosphenes (and vice versa), and superiorly located electrodes evoke inferiorly perceived phosphenes (and vice versa). Resolution appeared to be 1.5° of visual angle.

In another study a very similar drawing method was used to map phosphenes of sighted subjects with intractable epilepsy who were chronically implanted with subdural electrodes in the extrastriate visual cortex [21]. Subjects were asked to look at the center of a white board positioned 2 m away. The white board was divided into sections by horizontal and vertical median lines. The subjects were instructed to make drawings on a white paper, regarding the outline and location of the phosphenes. The paper was one tenth the size of the white board and had similar dividing lines. Phosphene shape, color and motion were also recorded. These drawings were then used to extract polar angle and eccentricity of the phosphene for mapping purposes. The results of this study showed that retinotopic maps could also be found on the lateral occipital cortex.

Several TMS studies made use of drawings of phosphenes made by subjects, starting with an early study by Marg and Rudiak using normally sighted people [24]. For optimal phosphene perception, subjects were seated in a darkened room with their eyes closed. Subjects made drawings of the phosphenes and reported on characteristics such as the shape, color, brightness/vividness and position and distance in the visual field relative to the fixation point. Besides detailed morphology of TMS-evoked phosphenes, the authors reported that phosphenes in the peripheral field of vision occur more frequently than central ones, in agreement with the findings of Ray et al. [27].

Subject drawings of perceived phosphenes were also applied in a TMS study with sighted subjects and two visually impaired subjects. One of the visually impaired subjects lost all functional vision at the age of 53 (subject was 61 years of age at the time of testing), while the other had a partial vision loss due to severe damage to the left striate cortex at the age of 8 (subject was in his early 40s when testing took place) [5]. Phosphenes were mapped by letting the subjects