Prime Minister Menachem Begin, who 12 days ago was Hospitalized with a mild stroke, continues to show "considerable improvement" and is expanding his official duties, his doctors said today.

The Hadassah Hospital said that Begin underwent comprehensive vision tests, which showed improvement in the eye dysfunction since he was hospitalized on July 19. The hospital said Begin held a long talk with Deputy Prime Ministers Yigael Vadin and several members of his staff. d

Although the medical team met to consider when Begin could be released, no decision was announced.

Previously aides said that Begin, whom doctors described as in "an advanced stage of recovery," probably would be released for home rest in about 10 days.

Begin suffer what doctors described as a "minor clot" in an artery in his brain. According to aides, he still suffers from occasional dizziness and loss of balance, although the most persistant symptom has been an impairment in vision.

Aides said that the 65--year-old prime minister apparently has lost some peripheral vision, which impedes his reading ability.

His doctors carefully have avoided using the term 'stroke' 'to describe Begin's condition, although they have said that in the strict sense it could be termed a very minor stroke.

Neurologists not connected with Begin's treatment have said that any stroke symptoms that do not pass after about six months generally are regarded as permanent disabilities, and that the principal danger of even a mild stroke is that about 20 per-cent of the victims suffer a recurrence or some other difficulty with the cardiovascular system.

Begin suffered a heart attack before his election two years ago, and afterwards suffered from an inflammation of membrane around the heart. Begin also suffers from diabetes.